Metazoa		Bivalvia
MOLLUSCA		BivalvE Glossary

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Bivalve Glossary

MOLLUSCA 
BASAL MOLLUSCA
|--Diasoma
|  |--Rostroconchia  
|  `--BIVALVIA  
|     |--Protobranchia  
|     `--HETERODONTA  
`--CYRTOSOMA
   |--CEPHALOPODA
   `--GASTROPODA 
      |--Eogastropoda  
      `--APOGASTROPODA
         |--Caenogastropoda
         `--HETEROBRANCHIA
            |--Opisthobranchia
            `--PULMONATA

Accessory plate: A secondary calcareous or corneous structure formed in some bivalves (e.g., Pholadidae) to protect the soft parts. In certain bivalves, a small shelly or partially shelly plate lying over the upper or lower margin, or a shelly plate attached to the internal ligament.

Accessory scar: In bivalves, a small, mostly circular impression on the inside of the shell made by other than the adductor and pallial muscles.

Adductor  ad-duc'tor  (L ad=to; ducere=to lead)  A muscle which draws a structure towards the medial line.  The major muscles (usually two in number) of the bivalves, which are used to close the shell.

Adductor muscle impression: Impression on inside of bivalve shell valves where the adductor muscle was attached. See Adductor Scar.

Adductor scar(s): The depressed scars of the muscles of a bivalve shell. A differentiated area on the interior of a bivalve shell that marks the attachment point of an adductor and is generally striated, depressed, or outlined. Attachment scars of adductor muscles inside a bivalve shell

Anisomyarian: Descriptive of bivalve shells with the scars of the two adductor muscles on the interior being of different sizes. The posterior scar is usually larger than the anterior.

Articulated   ar-tic'u-lated (L articulatus=jointed)   the union forming a joint as the interlocking teeth of the hinge plate in bivalves

Aurical or Auricle: Ear-shaped appendage (as in Pectinidae). Anterior or posterior projection along the hinge line of a bivalve shell.

Bead: A small, more or less hemispherical protuberance resembling a bead. Beads are smaller than nodules.

Beaded: Sculptured so as to resemble beads, or strings of beads.

Beak   (L beccus=a beak)  the rounded or pointed extremity (umbo) of a bivalve shell at which it began to grow. (oldest part of the valve)

Byssal a small opening or notch on the ventral margin for the passage of the byssus

Byssal orifice or notch: A triangular or rounded opening at the margins of certain bivalve shells i.e. Pectinidae for the passage of the protrusion of the foot.

Byssal sinus: Recess in the anterior of the valve of some bivalve shells to provide an outlet for the byssus notch.

Byssusbys'sus (Gr Byssos=a fine flax)  The fine fibers, or bundle of silky threads secreted by a gland found in the foot of some bivalves by which they attach themselves permanently to rocks or other solid objects; the bundle of fibrous, tough conchiolin strands secreted by the foot of some bivalves, passing out of the anterior end of the shell and use to secure the animal to a substrate.

Callum: Shell material filling the gap between valves in certain mature pholad bivalves.

Cardinal: In bivalves, situated more or less in the central part of the hinge area directly below the beaks. Elevated process on hinge plate near umbo usually with corresponding socket in opposite valve.

Cardinal teeth: The main central teeth in bivalve shells. Projections on the hinge line, located directly below the umbones.

Central teeth: In bivalves, the hinge teeth located on the interior under the umbones.

Dentition - teeth and sockets located at the hinge of the shell. The teeth guide the two valves into alignment when the valves close, interlock the valves to prevent shearing, and maintain contact between the valves during burrowing. The teeth fit into corresponding depressions on the opposite valve called sockets. see also Taxodont, Desmodont, Dysodont, Isodont, Schizodont, Heterodont, and Pachydont

Desmodont - type of dentition in which teeth are very reduced or absent with accessory ridges lying along the margin to take their place. This is a common form of dentition and all bivalves that possess it are infaunal suspension feeders.

Dysodont - type of dentition in which there are only small simple teeth near the edge of the valve.

Dorsal   dor'sal  (L dorsum=the back)  referring to the  back edge or top of a bivalve, in the region of the hinge.

Dorsal margin: In bivalves the hinge region.

Ear(s): Small extension of dorsal region of bivalve shell, usually with a notch between it and main part of shell, as in Pectinidae. In scallops and file shells, triangular or oblong projections at the ends of the hinge line.

Edentate: Without hinge teeth.

Edentulous: Lacking teeth.

Ententate: Without teeth; descriptive of a type of bivalve shell with a hinge that lacks teeth.

Equilateral: When beaks in bivalve are central. The anterior and posterior ends of each valve being of equal size.

Equi-sized: Equal in size.

Equivalve: When the two valves of a bivalve shell are of the same shape and size.

External ligament: That portion of the ligament visible when the valves of a bivalve are closed. See Ligament.

Foot  The muscular locomotor, undersurface of the body of a mollusk upon which the animal rests or moves.  In bivalves, the contraction and expansion of this organ is used in the burrowing, locomotion, or for anchoring the animal.

Gill chamber: In bivalves, the space between and around the gills.

Height: In bivalves the greatest distance from the beaks on a vertical line to the ventral margin.

Heterodont - type of dentition characterized by two or three small cardinal teeth below the umbo, as well as elongated lateral teeth anterior and posterior to these. The hinge teeth are differentiated into vertical cardinals below the beak (umbo) and sub-horizontal laterals on either side. Most Tertiary and Recent bivalves possess this type of dentition.

Hinge: Collective term for structures of the dorsal region of bivalve shells that function in opening and closing the valves.

Inequivalve: having the two valves (shells) dissimilar in shape and size.

Isodont: Descriptive of a bivalve shell in which the dentition consists of very large teeth located on either side of a central ligament pit; there is one pair of teeth and one pair of pits on one valve that articulate with two pits and two teeth on the other.

Isomyarian: Descriptive of a bivalve shell having the two adductor-muscle scars on the inner surface approximately equal in size.

Labial Palps   la'bi-al  palps  (L. labium=lip   palpare=to feel)  Paired ciliated triangular flaps on either side of the mouth in bivalves.

Lamellar gills   la-mel'lar gills  (L. lamella=small plate) Enlarged, flattened plate-like gills (ctenidia) which form the feeding organs of most bivalves. more

Lateral   lat'er-al (L  latus=the side0  pertaining to the side.

Lateral teeth; interlocking teeth of bivalves, not functioning as a hinge but serving to prevent valves from sliding upon each other when closed.

Left valve   Left valve of the shell where the dorsal edge or hinge is facing up and the anterior end is directed forward (away).

Ligament: In bivalves, a horny elastic structure or structures joining the two valves dorsally, mostly posterior to the beaks, and acting as a spring that causes the valves to open when adductor muscles relax, the external ligament being under tension, the internal (sometimes called "cartilage" or "resilium") being under compression. The ligament is usually situated slightly behind the embryonic shell (umbo). The ligament may be external (dorsal to the hinge) or internal (ventral to the hinge).

Ligament area: In some bivalves, the area between the umbones containing the external ligament.

Ligament cleft or pit: A narrow depression behind the umbones in which the ligament may lie.

Ligament shelf: A shelly internal or external platform on which the ligament lies.

Lunule: Curved or flat heart-shaped area on the outside of a bivalve shell along the hinge line and anterior to the umbones.

Monomyarian: Descriptive of bivalve shell with only one adductor-muscle scar on the inner surface of each valve.

Muscle scar: Slight depression on the inner surface of the valve of a bivalve shell where a muscle is attached; in limpets, the scar left by the margin of the mantle and the foot retractor muscles. See also Accessory Scar; Adductor scar.

Nacre   na'cre (Fr nacre=mothe-of-pearl)  The pearly or iridescent substance which lines the interior of some mollusk shells.

Nacreous: With a layer of nacre, or mother-of-pearl.

Opisthogyrate: In bivalves, having the beaks pointed backward, or posteriorly; opposite of prosogyrate.

Orbicular: Circular or rounded.

Orthogyrate: In bivalves, having the beaks pointed toward each other.

Pachydont - type of dentition in which the teeth are large, heavy, blunt and massive. This dentition occurs in rudists.

Pallial line: Impression or scar on the inner surface of a bivalve shell, parallel to the ventral edge, by the insertion of the muscles that attach the mantle to the shell. See also Mantle scar.

Pallial muscle: A muscle associated with the mantle and serve as point attachments.

Pallial sinus: An embayment of the pallial line, marking the space occupied by the retractor muscles for the siphons, hence always on the posterior part of the shell.

Prodissoconch The rudimentary or embryonic shell of a bivalve, the protoconch.

Radial: In bivalves, radiating from the umbones toward the ends and lower margins.

Radial lines: Radiating in relatively unbroken lines from the beaks of bivalves, as contrasted with concentric.

Radiating: Spreading in various directions

Reticulate: Lines, riblets, threads or grooves crossing each other like a net.

Rib: An elongate sculptural element of a shell, raised above the surrounding surface. A fine rib is a riblet, and a very fine rib is a thread.

Riblet: A small rib, but coarser than a thread.

Right Valve   Right half of the shell when the dorsal edge or hinge is facing up the anterior end is directed forward (away).

Schizodont - type of dentition (actinodont hinge) in which there are very large teeth with many parallel grooves normal to the axis of the tooth. All the individual teeth have denticulated edges. The Trigonioida have this form of dentition

Sculpture: the decoration (in terms of ridges, ribs, striae, etc) on the surface of a shell; a pattern of raised or depressed markings on the shell's surface. Synonymous with ornament.

Taxodont - type of dentition in which the hinge teeth are numerous and approximately equal, undifferentiated, and subparallel or radially arranged.

Thread: A very fine sculptural element of a shell, raised above the surrounding surface. Finer than a rib.

Truncate: Abruptly cut off; applied to the square-ended appearance of certain bivalve shells.

Umbo (pl. umbones)    um-bo  (L umbo=a knob or boss)  That point of a bivalve situated immediately above the hinge.  The upper (or earliest) part of a bivalve valve, as seen from the outside, the youngest, terminal portion of which is the beak, best seen in an interior view of the valve; the first part formed in a bivalve around which radial growth has proceeded. 

Umbonal cavity: The space inside valves within umbones and under hinge plate.

Umbonal reflection: When dorsal line is turned up and over and in front of umbo in each valve; found in Pholadidae.

Umbonal ridge: In bivalves, an angled or rounded ridge beginning at the umbo and usually extending to the hind end of a valve.

Uncinus (plural uncini): In gastropods, a marginal or lateral tooth on the radula, more or less hook-like in shape.

Undulate: Having a wavy surface.

Ventral   ven'tral  (L venter=the belly)   Of, pertaining to, or situated on, the lower side of a dorsoventral organism; the edge remote from the hinge in a bivalve;  the ventral margin is opposite the umbones.  Opposite to dorsal.

Abbreviations

(from Man and Mollusk Glossary)

AS       =          Anglo-Saxon
Fr        =          French
Gr        =          Greek
L          =          Latin

Links

Links

Glossary/Mollusks - a very comprehensive glossary

Man and Mollusc Glossary - includes derivation of original word