Section I. Chitones Regulares.--_anterior and Posterior Valves

of similar character._

A. Leptoidea.--Insertion plates obsolete, or, if present, unslit; Leptochiton, Hanleyia, Hemiarthrum, Microplax.

B. Ischnoidea.--Insertion plates sharp, smooth, fissured; with eaves; Trachydermon, Callochiton, Tonicella, Schizoplax, Leptoplax, Chaetopleura, Spongiochiton, Ischnochiton, Callistochiton.

C. Lophyroidea.--Insertion plates broad, pectinated, projecting backward; Chiton, Tonicia, Eudoxochiton, Craspedochiton.

D. Acanthoidea.--Insertion plates thrown forward; Sclerochiton, Acanthopleura, Dinoplax, Middendorffia, Nuttallina, Arthuria, Phacellopleura.