Introduction: Small trilobites (usually only a few mm long) with cephalon and pygidium similar in outline and size (isopygous); enrollment typical. It has been suggested that some Agnostida were planktonic.
Cephalon: cephalic shield with deeply parabolic outline, maximum width usually anterior of genal angle, sutures proparian or lacking; border convex; glabella fusiform, widest at base (except in Condylopygidae), glabellar segmentation highly variable, sometimes complex, but in some species entirely effaced; most species eyeless; hypostome natant, specialized with ribbon-like wings; rostral plate lacking (or uncalcified). 
Thorax: segments 2 (Agnostina) or 3 (some Eodiscina), axis typically broad, short fulcrate pleurae. 
Pygidium: strongly isopygous; pygidial margin typically closely matching cephalic margin. 
Occurrence: Cambrian Epoch 2 to Late Ordovician (Hirnantian). 
Suborders: Agnostina and Eodiscina. 

Cephalon: no facial sutures or eyes; cephalothoracic aperture present.
Thorax: 2 thoracic segments bearing distinctive articulating structures, but no articulating half-ring on anterior thoracic segment. 
Pygidium: axis usually wide, inflated, 3 or fewer segments, one of which usually carries a tubercle; pygidial margin often bearing posterolateral spines. Cuticle thin. 
Superfamilies: Agnostoidea and Condylopygoidea (each described below).

Superfamily Agnostoidea
Cephalon: with basal glabella lobes anteriorly directed, no clear occipital structure present, anterior glabellar lobe typically subequal in width to posteroglabella, or narrowing forward (not laterally expanded).
Thorax: as in typical Agnostina.
Pygidium: variable within bounds of typical Agnostina.
Representative Families: Agnostidae, Clavagnostidae, Diplagnostidae, Metagnostidae, Peronopsidae, Ptychagnostidae, Spinagnostidae
Representative Genera: Agnostus, Ammagnostus, Arthrorachis, Aspidagnostus, Clavagnostus, Diplagnostus, Poryagnostus, Glyptagnostus, Goniagnostus, Hypagnostus, Metagnostus, Oidalagnostus, Peronopsis, Phalagnostus, Pseudagnostus, Ptychagnostus, Spinagnostus

Superfamily Condylopygoidea
Cephalon: with transversely oriented basal glabella lobes, separated by medial plate, together forming a clear occipital structure, anterior glabellar lobe laterally expanded around anterior end of posteroglabella, sometimes separated by a median sulcus.
Thorax: as in typical Agnostina.
Pygidium: axis with triannulate anteroaxis; broad, posteriorly rounded posteroaxis.
Representative Families: Condylopygidae
Representative Genera: Condylopyge, Pleuroctenium.

Two families of Agnostina, Phalacromidae and Sphaeragnostidae, can not clearly be placed within a Superfamily. 

Introduction: There is some controversy over the placement of Eodiscina in the Agnostida. Presence of eyes, sutures, and typical thoracic structure suggest that Eodiscina may be more akin to (or a suborder of) Ptychopariida, but for now the overall similarity to Agnostina and (placement in Agnostida) prevails in the 1997 systematic treatment of the current Treatise.
Cephalon: with proparian sutures, when present; some retain eyes; glabella usually simple, narrow and tapering. 
Thorax: 2 or 3 thoracic segments of normal trilobite form, with articulating half-rings 
Pygidium: axis can be long and narrow, extending close to margin, generally distinctly divided into more than 3 ring-like segments; pleural region segmented in some species, sometimes pygidial segmentation effaced; border furrows often very deep; axis sometimes spinose. 
Superfamily/Families: Eodiscoidea/Calodiscidae, Eodiscidae (including Pagetiidae), Hebediscidae, Tsunyidiscidae, Weymouthiidae, Yukoniidae. 
Representative Genera: Calodiscus, Eodiscus, Hebediscus, Pagetia, Tsunyidiscus, Weymouthia, Yukonia.