8 
SHELL GALLERY. 
head and crawling foot like a true Limpet, and a posterior anal 
orifice. These several anatomical peculiarities at one time induced cer- 
tain eminent authorities to hesitate in considering them molluscs ; but 
now that the development from the egg has been investigated, their 
association with the Mollusca may be considered definitely settled. 
Chitons are found in ail parts of the world, the finest inhabiting 
tropical countries. They live chiefly on rocks and under stones at 
low-water or at moderate depths ; but a few forms have been dis- 
covered by the ' Challenger ' Expedition at depths exceeding 2000 
Coat-of-mail Shells, or Chitons. 
1. Chiton squamosw (upper surface). 
2. Chiton elegans (lower surface) : a, mouth ; 6, foot ; c, mantle ; d, gills. 
fathoms. The numerous sections of the group are principally dis- 
tinguished by differences in the edges of the plates or valves which 
are inserted in the mantle, and in the different kinds of ornamenta- 
tion upon the upper surface of the mantle-border. This, in some 
species, is quite smooth, in others covered with a dense mass of 
minute grains or scales, and in others armed with short prickly 
spines. In the giant Cryptochiton of Kamtschatka the plates are 
entirely covered over by the thick leathery granular mantle, and in 
another set, Cryptoplax, which consists of long slug-like animals, the 
plates are very small, and placed at intervals along the back. 
About five hundred living species are known, and about one- 
