E. Schizoidea.—Posterior valve fissured; Lorica, Schizochiton.
F. Placiphoroidea.—Posterior valve unslit, internally ridged, umbo nearly terminal; Enoplochiton, Ornithochiton, Plaxiphora.
G. Mopaloidea.—Posterior valve with posterior sinus and one slit on each side; Mopalia, Katherina, Acanthochiton, Notoplax.
H. Cryptoidea.—With double sutural laminae; Cryptoconchus, Amicula, Cryptochiton.
I. Chitonelloidea.—Posterior valve funnel shaped; laminae thrown forward; Chitonellus, Choneplax.
Fig. 265.—Chitonellus fasciatus Quoy; ant, anterior end.
Sub-order 2. Aplacophora.—Animal vermiform, foot absent, or a mere groove, cuticle more or less covered with spicules.
According to Marion, one of the principal authorities on the group, the Aplacophora are perhaps Amphineura whose development has been arrested at an early stage, their worm-like exterior being due to adaptation to surroundings. They have hitherto been found chiefly in the N. Atlantic and Mediterranean, generally at considerable depths, and often associated with certain polyps in a way which suggests a kind of commensalism.
Fam. 1. Neomeniidae.—Foot a narrow groove, intestinal tube without differentiated liver, kidneys with common exterior orifice, sexes united, ctenidia present or absent. Genera: Neomenia (Fig. [266]), Paramenia, Proneomenia, Ismenia, Lepidomenia, Dondersia.