8 



SHELL GALLERY. 



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 MolUisca 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 



Fiff. 2. 



Coat-of-mail Shells, or Chitons. 



1. Chiton squamosus (upper surface). 



2. Chiton elegans (lower surface) : a, mouth ; h, 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 GryptocMton of Kamtschatka the plates are 

 entirely covered over by the thick leathery granular mantle, and in 

 another set, Gryptoplax, which consists of long slug-like animals, the 

 plates are very small, and placed at intervals along the back. 



Five or six hundred living species are known, and about one- 



