Palaeos:		Glossary
The Vertebrates		A

Glossary: A

For most phrases beginning with directional words, e.g. "posterior," "dorsal," "external," etc., or some generic anatomical terms, e.g., "vena," look under the next word in the phrase. However, note that this convention is not used with complete consistency in this Glossary.

Other Web Glossaries and Related Lists

Anatomical Dictionary (One of several useful pronunciation and definitional guides from Jeff Poling's site)
Laboratories
GLOSSARY (general anatomy)
Greek and Latin Roots and Terms (what it says)

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

- A -

A/paracone: in symmetrodont upper molars, the central (lingual) cusp of the trigon. This is home-made terminology used to reflect the fact that cusp homologies are unclear in symmetrodonts so that some workers use only a letter designation. However, many others traditionally refer to this cusp as the paracone.

Abducens nerve: cranial nerve VI.  The abducens enervates the lateral rectus muscle of the eye, which rotates the eyeball laterally.  See figure at rectus muscles; see also discussion and figures of the gnathostome orbit.

Abduction: in tetrapod locomotion, rotation of a limb upward, in a vertical plane.  Opposite of adduction; and in contrast to protraction or retraction (movement in a horizontal plane) or to rotation about the long axis of a limb. C.f. Humerus.

Abductor L. ab = away from, and ducere = to lead. These muscles (and nerves) are named for their function. For example, the abducens nerve is named because it turns the eyeball outward.

Abo-Cutler Formation: Late Carboniferous to Early Permian of New Mexico. The Abo Reef underlies the Early Permian Cutler fm of West Texas. One can only speculate that this is the same.

Abomasum: the fourth chamber of the ruminant digestive tract, the second of two chambers used for bacterial fermentation of cellulose. See Artiodactyla for figure.

Aboral: in a direction away from the mouth cavity.  This slightly ambiguous term is used to mean the opposite of occlusal in some cases where the side of the tooth opposite the biting surface is not a root, as in the tooth plates of chimaeriforms, or other "bradydont" dentitions in which spent teeth are retained in the dentition as, e.g., part of a pavement dentition.

Abrahamskraal Formation: Late Permian of South Africa. Part of the Tapinocephalus Assemblage Zone of the Beaufort Group. Elliotsmithia, therapsids.

Acellular: without cells; e.g. "acellular bone" is bone that is not supported by or contain living cells.

Acetabulum: the socket in the pelvis for the head of the femur, normally at the junction of the pubis, ischium and ilium. See figure at antitrochanter.

Acicular: Needle-shaped; slender like a needle or bristle, or having a needle-like point.

Acinaciform: shaped like a scimitar (or cutlass, or even saber).  

Acoelous: or "amphiplatynate" vertebral centra flat on both ends -- neither procoelous (anteriorly concave & posteriorly convex) or opisthocoelous (vice-versa). Same as platycoelous.

Acoustic meatus, internal: on posterior surface of petrosal (or petrous portion of temporal); transmits facial nerve, auditory nerve (vestibulocochlear or VIIIth nerve), & labyrinthine artery from brain to inner ear.  

Acrocoracoid process: in neornithine birds, the distinctive hook-shaped process at the proximal end of the coracoid that forms a part of the triosseal canal.  Actually, the process is not restricted to birds and isn't hook-shaped in, for example, ducks.  But such details are tedious ...

Acrodont: Having teeth attached to the edge of the jawbone without sockets.  See Tooth Implantation.

Acromioclavicular joint: The acromioclavicular joint is located between the acromion (a projection of the scapula that forms the point of the shoulder) and the clavicle (the collar bone). This is a gliding type of joint.  

Acromion (or Acromial) (Process): the outer end of the spine of the scapula that forms the outer angle of the shoulder, and articulates with the clavicle. Etymology: Gr. ακρoν (akron) = tip or summit, and oμoς (omos) = shoulder. The word ακρoν gives the combing forms acra- or acro-. The Acropolis in Athens was built on a summit. See figure at supraspinous fossa.

Acuminate: tapering (usually gradually) to a sharp point.

Adduction: in tetrapod locomotion, rotation of a limb downward, in a vertical plane.  Opposite of abduction; and in contrast to protraction or retraction (movement in a horizontal plane) or to rotation about the long axis of a limb. C.f. Humerus.

Adductor: L. ad = to, toward, and ducere = to lead. These muscles (and nerves) are named for their function. For example, the all-important jaw adductors are muscles that move the jaws together.

Adductor blade: longitudinal ridge on the femur of Elpistostegalia and basal Tetrapoda.  "The term 'adductor blade' ... is used ... to distinguish the prominent ridge bearing fourth and internal trochanters, from the more acute, and what appears to be primitively short, adductor crest."  Coates (1996).  The adductor crest is continuous with the distal end of the blade.  See image of Acanthostega hindlimb.  

Adductor fossa: The opening in the palate which in life contained the adductor (jaw closing) muscles. Not to be confused with the interpterygoid vacuities, choanae or other holes in the palate. The image at right, from Damiani (2001), shows a temnospondyl palate with the various fossae labeled.

Adipose: fat.

Admiral Formation: See Stanton Formation.

Adsymphysial: same as parasymphysial, i.e. flanking the symphysis (the point at which the two halves of the jaw meet).

Aegithognathous: type of avian palate with vomer broad and truncate anteriorly. Maxillopalatines do not join but do touch basisphenoidal rostrum. e.g., Passeriformes.

Aeolian: of sediments, deposited by wind, as in desert sand dunes.

Aina Dal Formation: Famennian of East Greenland, Celsius Bjerg Group, above Elsa Dal and below Wimans Bjerg Fms.  Ichthyostega  (Acanthostega also present, but rare).  "The Aina Dal Formation reaches a maximum thickness of 90 m at its type section on Stensiö Bjerg and towards Paralleldal (Olsen and Larsen 1993). The formation consists of dark red to grey or black fine-grained sandstones and siltstones, representing a variably active meandering river and flood basin environment. The Wimans Bjerg Formation conformably overlies the Aina Dal Formation, and is, apart from trace fossils, almost unfossiliferous."  Blom (2005).  

Alar process: generic term for a wing- or fan-like projection.

Albedo: a surface property of a material which measures the proportion of incident light is reflected, rather than absorbed or transmitted.  Albedo varies with wavelength and angle of incidence.  Unless these are specified, the term refers to an average reflectivity across the visible light spectrum and assumes normal incidence (i.e. that the incident light strikes at an angle of 90° with the surface).  The effective albedo of a surface also depends on its texture and contour, since some incident light may be reflected onto other portions of the surface.  

Albian: the last age of the Early Cretaceous (Early Cretaceous II), about 112-99 Mya.

Alisphenoid: epipterygoid bone in mammals.

Alisphenoid Canal: see dog_orbit.jpg. I'm a bit vague on whether this is really the "maxillary." In any case, it appears to be just below the middle ear.  In somewhat more rational animal like insectivores, the alisphenoid canal consists of a foramen in the alisphenoid exposure on the basicranium, near the foramen ovale which communicates with the cavum epiptericum just behind its opening into the orbitotemporal region.  Asher et al. (2002).  

Allantois: one of the membranes of the amniotic egg, it provides a surface for gas exchange and waste removal.

Allotype: A term, not regulated by the ICZN Code, for a designated specimen of opposite sex to the holotype.

Alternating diplospondyly: see diplospondyly.

Alula: the small digit 1 (thumb) which emerges from the proximal base of the carpometacarpus in birds.

Alveolar shelf: the tooth-bearing palatal shelf of the marginal bones of the jaws.

Allometry: roughly speaking, proportionality at different sizes, but sometimes used with respect to age, temperature, or any other variable. A structure is said to exhibit "positive" or "negative" allometry if it becomes larger more or less than in direct proportion to size. For example the human head grows with age, but becomes a smaller proportion of overall body size with age and development. Therefore we might say (assuming that we wished to be unusually difficult to understand) that the "cranium exhibits negative allometry with ontogeny."

Ameloblast: a specialized epithelial cell type responsible for enamel deposition.  See Teeth.

Amelogenin: the protein which appears to be largely responsible for forming the matrix in which enamel is formed.

Aminadav Formation: Late Cretaceous (Cenomanian) of Israel, Judea Group. Includes the 'Ein Yabrud limestone quarry, type locality of Haasiophis.

Amnion: the innermost layer of the amniotic egg, it retains a fluid which surrounds the embryo.

Amniotic egg: see Introduction to the Amniota.

Amphiarthrodial: slightly moveable joints, such as between vertebrae, as opposed to freely moveable (diarthrodial) or immoveable (synarthrodial), such as the knee and sutural joints, respectively.

Amphicoelous: vertebral centra concave at both ends. lab7 photos

Amphistylic: a form of jaw suspension (e.g. in basal gnathostomes, other than placoderms) in which jaw is suspended both by the hyomandibula and by a direct connection between the jaw and the braincase. Introduction to the skeletal system

Amphiplatinate: see acoelous.

Ampulla L. ampulla = a jug. Perhaps an onomatopoetic word whose sound suggests the object; in this case, fluid flowing from the jug. Applied in anatomy to a number or structures supposedly resembling a jug, such as the ampullae of the labyrinth in the inner ear. See The Ear.

Anacanthous: of fishes, lacking dorsal fin spines. Opposite of phalacanthous.

Anacleto Formation: Early Campanian of Argentina.  Overlies the Bajo de la Carpa Fm. and underlies the Allen Fm.  Part of the Río Colorado Subgroup, Neuquén Group, Neuquén Basin.  Braided stream and floodplain deposits with soils.  Fossiliferous with dinosaurs, fishes, crocs, birds, lizards & turtles.  Apparently east, or forming part, of the Andes foreland Basin but also just west of the South Atlantic.  

Analogy: see homology.

Anamestic: of fish bones, a dermal bone which forms from ossification of embryonic membranes without nucleating around a sensory canal.  Anamestic bones tend to be small, irregularly shaped and show considerable variability between individuals.

Anapophysis: in some mammals, the articular surfaces of the vertebrae are on the neural arches, rather than on the centra.  The anapophyses are the posterior articulations, analogous to the postzygapophyses.  Frequently both structures are present and in close proximity.  The corresponding anterior articulations are the metapophyses.  

Anastomosis Gr. ana = up, and stoma = mouth; hence an opening up. (Don't take this etymology too seriously ...)

Anconal: [1] in medical terminology, relating to the elbow. [2] as a directional indication, looking towards the elbow.  As far as we can make out, this is another antonym for palmar, and so synonymous with "dorsal" in the special sense of that word as it applies to limbs.

Anguilliform: [1] eel-like in shape. [2] eel-like in locomotion, in which the body subscribes more than 1 sine wave at any given time and the there is a perceptible "traveling wave" down the body during locomotion.  As opposed to, e.g. thunniform or carangiform locomotion in which a limited posterior segment moves back & forth (or up & down in mammals).  Some useful detail at DESIGNING AN UNDERWATER EEL-LIKE ROBOT AND DEVELOPING ....

Angular process: in mammals, the most ventral of the three proximal processes of the jaw.  See image at coronoid process. 

Angulated: bent, V-shaped.  This useful term isn't used much.  It generally refers to the shape of an otherwise plate-like bone with a relatively sharp bend in the middle, like the covers of a partly open book, or an open laptop computer. 

Anisian Age: The first Age of the Middle Triassic (242-234 Mya). 

Anisodactyl: In birds, the basic digital configuration in which digit 1 (the toe or hallux) points posteriorly.

Ankylosed thecodont:   See Tooth Implantation.

Ankylosis: ontogenetic fusion of bones. That is, a bone fusion that occurs after birth. Often used of pathologically fused bones in medicine, but a normal biological process in many organisms including humans.

Annulus (pl. annuli). L. anus = ring + diminutive ulus. Hence a little asshole. Applied to any ring or ring-shaped structure, not merely to junior siblings.

Anocleithrum: a relict member of the supracleithral series (bones dorsal to the cleithrum in fishes), found in some early tetrapods.  The anocleithrum is a small, oval bone, normally dorsal and medial to the cleithrum and articulating with it.

Ant: (abbr.) anterior

Anterior trochanter: [1] (of the femur) probably the same as the lesser trochanter.  [2] (of the fibula) probably the same as the illiofibularis tubercle.

Anticlinal: in geology, sloping away from a common center or peak. More generally, having elements which bend away from a common center. Opposite of synclinal.

Antimere: the opposite member of a paired structure, e.g. flapping flight requires the coordinated exertion of each wing with its antimere.

Antimeric: relating to an antimere.

Antitrochanter: a tuberosity or ridge contained in the acetabulum. MUSEE. DINOSAURES - LES COLLECTIONS. See Figure. This definition is quite different from the usage of Romer, who states: "Above the acetabulum the upper margin of the ilium was thickened ... . In hadrosaurs and ceratopsians there develops a downward projection from this thickened area, the antitrochanter, from which the iliofemoralis muscle probably took origin." Romer (1956: 326). Romer's definition is probably the only correct usage.

Antorbital fenestra: a hole in the skull just in front of the orbit. See figure at maxillary fenestra.

Antorbital fossa: a depression in the skull anterior to the orbit. The fossa is frequently excavated all the way through the bone, at least in part, to form an antorbital fenestra (q.v.) as well as other fenestrations.

Antotic process: of the ethmoid or sphenethmoid.  In sarcopterygians (and perhaps other fishes), a process of the braincase located posterodorsal to the orbit which articulates with the ascending process of the palatoquadrate (= epiptyerygoid) as part of the dorsal jaw articulation.  See Diplocercides for gigantic image.  

Antrum: Gr. antron = a cave. A cavity.

Apatite: A class of minerals including several incorporated in the teeth and/or scales of vertebrates.  The basic unit of apatite has peculiar chemical formula sometimes given as: Ca₅(PO₄)₃(OH, Cl, F).  To understand what's going on, its best to start with a simple block of calcium phosphate: CaHPO₄.  This has a very simple structure in which the two positive charges of the calcium ion, Ca⁺⁺ are balanced by the two negative charges of the phosphate ion, in the form at which it exists at physiologically relevant pH levels: HPO₄¨¨.   In apatite, some of the phosphate groups are replaced by another ion, one with only a single negative charge.  This typically results from the inclusion of hydroxyl (OH ¨ ), chloride (Cl ¨ ), or fluoride (F ¨ ) ions.  If the inclusion is a hydroxyl ion, the resulting material is hydroxyapatite (except in Europe, where it becomes hydroxylapatite), the mineral constituent of bone, including dentine and enamel.  In order to maintain charge neutrality, two things happen.  First, some of the charges are "shared" between adjacent cells of the crystal.  Second, the phosphate is forced to behave as a trivalent ion (an ion with three negative charges), equivalent to phosphate at a much higher pH, i.e., PO₄¨¨¨.  The physical properties of the resulting material make it uniquely useful for living organisms.  The cross-linking of crystal cells through charge-sharing make apatite extremely strong.  The actual alignment of the ions in the crystal is partially planar, but adjacent planes are rotated at 60° angles to each other, which means that the bulk material is highly resistant to shearing -- a very important attribute for teeth.  The substitution of a small, monovalent ion for phosphate also leaves a regular series of physical "holes" in the crystal structure, which contributes to three additional important properties.  (1) It provides room for organic components of the matrix to approach the ions and bind closely to the mineral by polar interactions.  (2) For the same reason, enzymes can "reach inside" the crystal to dismantle it, allowing bone to be reworked and reshaped.  It also helps that the phosphate ions in the crystal are held in a trivalent state.  All it takes to dismantle the structure is the introduction of hydrogen ions from water in exactly the right places to unlock the crystal structure.  (3) Finally, the presence of "holes" in the crystal gives the bulk material some compressibility, so that it can adapt flexibly to compression rather than shattering.  In short, apatite exhibits a remarkable combination of strength, hardness, flexibility, and biochemical reactivity.

Aphetohyoidean: a condition involving "the presence of a non-suspensory hyoid arch behind a full post-mandibular gill slit." Stahl (1988: 858). This is one presumed primitive condition for the jaw suspension of gnathostomes.

Apical: in mammalian dentition, toward the crown.

Aplesodic: of a fin, the condition in which the basals and radials do not reach to the distal margin of the fin.  This may refer to very primitive fish which have no support for the distal fin, or to highly derived fish which may have ceratotrichia or other non-bony support for the distal fin.  Opposite of plesodic.  

Apocrine glands: sweat glands associated with hair cells which secrete sweat containing complex organic compounds often having a characteristic odor. In humans, these glands become active at puberty and the odors may reflect emotion, state of health, sexual identity and maturity, and diet. Compare eccrine glands.

Apomorphy: a character state which is unique to a single, terminal taxon. Example: among primates, complex grammar is an apomorphy of human beings. It is quite diagnostic of humans, but useless in determining phylogenetic relationships because it is not a shared, derived characteristic, or synapomorphy, of any larger group.

Aponeurosis: a sheet- or ribbon-like tendinous expansion, serving mainly to connect a muscle with the parts that it moves.  The best example is perhaps "palmar aponeurosis," the dense sheet of tendons underlying the palm in humans and many other tetrapods.

Appress: to be in contact with.

Apteria: areas on the skin of the embryonic bird which do not develop feather primordia.

Aptian: an age of the Early Cretaceous (mid-Cretaceous) about 121-112 Mya.

Archenteron: the internal body cavity formed by gastrulation. The cells lining the archenteron develop into endoderm. See Early Development Terms.

Arcocentrum: in elasmobranchs, the cartilaginous arch and its base in the vertebrae. Dictionary of Ichthyology.  I have also seen this used of actinopterygians.  Poyato-Ariza & Wenz (2002).  

Arctometatarsalian: condition in which proximal half of metatarsal III is thin, splint-like, or even absent and closely appressed by metatarsals II and IV. The distal portion of metatarsal III is hollow (as are II and IV) and typically longer than II and IV. The functional significance of this arrangement is not completely clear. Likely, the entire metatarsus was bound tightly and the condition served to transmit force evenly over the metatarsals and aid in running. Holtz (1995).

Arcual plate: oblong, denticulated palatal bones which covered the anterior end of the notochord in some sarcopterygian groups.  Also referred to as parotic plates.  

Arcualia: "There are primitively two pairs of [metamerically arranged endoskeletal] elements in each metamere and on each side [of the notochord]: the interdorsals and basidorsals. In the gnathostomes, there are two additional pairs ventrally to the notochord: the interventrals and basiventrals. These elements are called arcualia and can fuse to a notochordal calcification, the centrum. The ensemble of the arcualia + centrum is the vertebra, and the ensemble of the vertebrae is the vertebral column." See Vertebrata (Phillipe Janvier).

Arcuate: in the shape of a smooth arc; not straight or broken line. 

Arenig Age: Formerly, the second age of the Early Ordovician Epoch.  Includes the current Floian Age of the Early Ordovician, as well as the Dapingian and early Darriwilian Ages of the Middle Ordovician. Approximately 488 to 465 Mya. 

Argillaceous: Describing rocks or sediments containing particles that are silt- or clay-sized, less than 0.625 mm in size; any sediment containing large amounts of clay.

Articulated: of a fossil, a condition in which the bones are still contacting the each other more or less as they would in life.

Arundel Clay: according to Cifelli et al. (1999), a member of the Patuxent Formation. According to the Maryland Geological Survey, it conformably overlies the Patuxent Formation in the Potomac Group.  Early Cretaceous II (Albian) of Maryland, USA.  Dark gray and maroon lignitic clays; abundant siderite concretions; present only in Baltimore-Washington area.  Oxbow swamps. Deinonychus, Tenontosaurus, Acrocanthosaurus, and triconodonts. 

Ascending process of the palatoquadrate: the epipterygoid -- termed ascending process when enough of the palatoquadrate is present to think of it as a process.  Under either name, it forms an articulation between the upper jaw and the braincase (and/or the skull table).  See  pterygoid for more context and figure.

Aspect ratio: in wings, the ratio of length to mean width for wings of regular shape. For irregular wings, other formulae apply. Wings with high aspect ratio indicate high-speed, long distance fliers. Such wings are less useful for low-speed flying or high maneuverability because they generate relatively little lift. To back off from jargon for a moment, a wing keeps the critter in the air because it pushes down on the air. At high speed, even a narrow wing passes over (and so pushes down on) a lot of air during the power stroke. At low speed, it takes a lot more area to push on the same amount of air. Length of wing helps, since a long wing pushes down further for each degree of downstroke. However, a long wing creates other problems: (a) it takes a lot of muscle to move a wing-tip out at the end of a long lever arm; (b) a tiny change in attitude at the end of a long lever arm can make a big difference in exactly how the airstream is directed. The bottom line is that high aspect ratio wings are for strong animals flying fast and high who don't need to be troubled too much about precision maneuvers or the sudden changes in wind speed and direction found near the ground.  

The same general principles apply to the caudal fins of fish and the expanded tails of aquatic tetrapods.  However, these principles are less easy to correlate with design and function in water.  As an empirical matter, caudal fins of low aspect ratio are usually found in "unsteady swimmers" -- such as ambush predators using very high acceleration lunges with no sustained swimming.

Aspidine (= aspidin): When all is said and done, aspidine is probably a generic term for acellular bone, as it occurs in early vertebrates. Janvier (1996: 84-85, 95). When fully elaborated, it is typically three layered. The outer layer is the dentine cap. The large middle layer is cancellous, with straight vertical walls enclosing large spaces. The thin inner layer is dense and lamellar with no included spaces. Janvier also states that aspidine is characterized by "incremental growth zones, as well as some peculiar fibre-like lineaments." As with most terms for bone-like substances, the word (in this sense) refers to a structural type, rather than a distinct chemical entity. Unfortunately, there is considerable structural variation in the materials referred to as "aspidine," making the foregoing definition -- or any definition -- somewhat suspect. Either the middle or outer layer may be missing or replaced by other material, as shown for psammosteidans in the figure. In psammosteidans, the cancellous middle layer is replaced by a thick layer of amorphous "trabecular aspidine" as shown in the figure.

Aspidospondyly: a condition in which all vertebral elements (centra, arches) remain as separate units. Opposite of holospondyly.

Aspondyly: the condition of having no vertebral centra.

Astragalus: One of the two proximal tarsals or upper ankle bones. It is the more medial of the two and usually articulates with the tibia. Compare calcaneum. See Figure at Tarsus.

Astragalus, ascending process: the ascending process of the astragalus is just like it sounds.  The inner (medial) ankle bone (the astragalus) sends a process up the shaft of the tibia.  The ascending process is normally fused to the tibia.  

Astragalar foramen: the opening of a canal on the proximal surface of the astragalus through which a nerve and vessels pass in primitive mammals.  

Atlantal: relating to the atlas.  

Atlas: Gr. Atlas was the mythological Titan who supported the world on his shoulders. Vesalius, in the sixteenth century, gave this name to the first cervical vertebra of tetrapods, which articulates with the skull, normally via condyles which permit the skull to move dorsoventrally.

Auditory bulla: the "inflated"-looking osseous covering of the middle ear and the floor of the skull in that region.

Auditory meatus, external: the "ear" of conventional speech; the outer ear; a passage leading from the environment to the tympanic membrane, often shaped so as to gather and concentrate sound from a particular direction.  See Ear

Auditory meatus, internal: in mammals, the common foramen for the VIIIth (auditory) and VIIth (facial) cranial nerves. Kermack et al. (1981: 97). "an opening on the posterior surface of the petrous portion of the temporal bone through which the auditory and facial nerves pass." EPIC Glossary

Auditory ossicle: a small bone used to conduct sound energy. The term is normally used to refer to the mammalian complement of malleus (articular), incus (quadrate) and stapes. It applies equally well to the columella (hyomandibula) of many other tetrapods and the weird assortment of bones in the Weberian organs of various teleost groups.

Aulacodont:   See Tooth Implantation.

Autapomorphy: a character which is unique to a particular taxon.

Autodiastyly: a form of jaw suspension in which the palatoquadrate is suspended from two articulations with the braincase.  This may be the original form of jaw suspension.  See discussion at Holocephali.

Autogenous: of bones, separate, not fused.

Autopalatine: an endochondral bone consisting of the anterior portion of the palatoquadrate (primitive upper jaw).  See image and additional information at pterygoid.  

Autopodium (autopod): the manus or pes, including digits (phalanges), metacarpals or metatarsals (metapodium).  May or may not include the carpals or tarsals (mesopodium).  

Autostylic: a form of jaw suspension (e.g. in lungfishes and in stem tetrapods) in which the upper jaw (palatoquadrate) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibula.

Autotomy: in Lepidosaurs and related forms, an animal can frequently escape predators by allowing part of its tail to break off. This is referred to as autotomy.

Axial: toward an imaginary axis running antero-posteriorly through the middle of the organism or structure; central. Opposite of radial.  In almost all cases, axially means the same as anteroposteriorly or longitudinally.  

Axilla: arm pit.

Axillary foramen: in some antiarch placoderms, an opening in the anterior ventrolateral plate which presumably allowed nerves and blood vessels to communicate with the pectoral appendages ("arms"). See figure.

Axis: the second cervical vertebra of terrestrial vertebrates; rotary movements of the head occur between the atlas and axis. Fr L. axis = axle or pivot.

Axonost: pterygiophore (the cartilage or bone on the outer end of which sit the median fin rays or spines), sometimes the proximal pterygiophore

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

checked ATW050108