26 



SHELL GALLERY. 



the back like a sea-worm, symmetrical organs of reproduction on 

 each side like the bivalves, a head and crawling-foot like a true 

 Limpet, and a posterior anal orifice like the Keyhole Limpets. 



Fig. 14. 



Coat-of-inail Shells, or Chitons. 



1. Chiton squamosus (upper surface). 



2. Chiton elegans (lower surface) : a, mouth ; b, foot; c, mantle ; d, gills. 



These several anatomical peculiarities at one time induced certain 

 eminent authorities to hesitate in considering them mollusks ; 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 all 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 

 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 kiridsof ornamentation 

 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 w r ith short prickly spines. In the 



