Six or eight species are exceedingly common upon the west [pg323] coast. The collector who has successfully taken a number of these will not fail to note the varying degrees of width of the girdle in the different species. In some cases the girdle partially covers the overlapping shell plates. The largest of all the chitons inhabits the California shores and is known as ^Cryptochiton stelleri^. It measures fully six inches in length and three in width. In /Cryptochiton/ the girdle is continued over the shell plates as a tough, gritty, brownish covering. The shell plates are white, and, as detached pieces, are frequently to be found cast up upon the California beaches, where, from their curious shape, they are often referred to as "butterfly-shells." [Illustration: /Chiton ruber./ /Chiton apiculatus./] On the east coast of America there are comparatively few species, and these are all small and inconspicuous. The three commonest ones are ^Chiton ruber^, ^C. albus^, and ^C. apiculatus^. They may readily be found on stones or dead shells fished up from below low-tide mark, or in rock-pools left by the receding tide. ORDER =APLACOPHORA= This second order of the /Amphineura/ is entirely destitute of the shell plates characteristic of the first. The outer mantle surface, however, develops a number of calcareous granules which correspond to a shell. The /Aplacophora/ are symmetrical, having the mouth and the excretory opening at the anterior and posterior ends respectively. The foot is greatly modified, being narrowed at times to a mere slit along the ventral surface. Some of them have long, worm-like bodies. This is no doubt a very primitive form of mollusk, or, if not a primitive form, it has greatly degenerated from some higher type. They are supposed to be commensal in habit and live in the deeper water offshore. The /Aplacophora/ are not littoral animals, and it is extremely unlikely that one will ever encounter them upon the beach, unless, possibly, after the severest of storms. We may therefore leave them with this mere mention and proceed to the second class of mollusks, the /Gasteropoda/. [pg324] TABLE SHOWING THE CLASSIFICATION OF THE GASTEROPODS AND SCAPHOPODS DESCRIBED IN THIS CHAPTER Phylum =MOLLUSCA= Class =GASTEROPODA= Order =OPISTHOBRANCHIATA= Suborder =TECTIBRANCHIATA= Families Genera Species BULLIDÆ ^Bulla^ ^B. (Haminea) solitaria^ ^B. occidentalis^ ^B. nebulosa^ APLYSIIDÆ ^Aplysia^ ^A. Wilcoxii^ Suborder =NUDIBRANCHIATA= DENDRONOTIDÆ ^Dendronotus^ ^D. arborescens^ ÆOLIDIDÆ ^Æolis^ ^Æ. papillosa^ DORIDIDÆ ^Doris^ ^D. bilamellata^ Order =PROSOBRANCHIATA= Suborder =DIATOCARDIA= Families Genera Species