MOLLUSKS 323 
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 over- 
lapping 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 gir- 
dle 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.” 
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. 
Chiton ruber. Chiton apiculatus. 
ORDER APLACOPHORA 
This second order of the Amphineura is entirely destitute of 
the shell plates characteristic of the first. The outer mantle sur- 
face, 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 degen- 
erated from some higher type. They are supposed to be com- 
mensal in habit and live in the dgeper water offshore. 
The Aplacophora are not littoral animals, and it is extremely 
unlikely that one will ever encounter them upon the beach, un- 
less, 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. 
