OF MOLLUSCA. 179 



phala. The veil is often absent in other Mollusca, or only appears 

 as oral or labial tentacles. 



The valves are not formed by four united joints ; and still less 

 do they support the opinion that the hinder valve is the true 

 shell, analogous to Patella, and the anterior valves supplementary 

 on it. 



It has been the habit of conchologists to keep the valves of this 

 genus attached to the dry mantle of the animal, and to regard them as 

 a single shell ; but it should be recollected that the separate valves 

 are the analogues of the only parts of the molluscous animals which 

 are usually kept in cabinets and studied by conchologists. 



It has been objected that the character derived from the form 

 of the plate of insertion of the valves can only be seen by the de- 

 struction of the specimens as they are usually kept in cabinets, but 

 they can generally be seen from the under side, or through the sub- 

 stance of the mantle ; when this is not the case, the form of the 

 plate of insertion can be easily developed by paring away the under 

 surface of the mantle, so as to show part of the edge of the valves, 

 without injury to the specimen ; and they may be easily made more 

 visible through the inner side of the mantle by being soaked for a 

 few hours in a weak solution of caustic potash ; but care should be 

 taken that the specimen is not left too long in soak, nor the solution 

 be too strong, otherwise the margin will be dissolved. 



The form of the plate of insertion may also be easily predicted by 

 inspecting the inner surface of the valves, for the notches in the 

 margin leave an impressed line from the vertex of the valve, as they 

 are gradually filled up by the growth of the valve. 



The valves are best separated from the coriaceous skin of the 

 body, called the mantle, by soaking them in a strong solution of 

 caustic potash ; as then the plates of insertion are cleaned, and not 

 broken, which they are likely to be if they are taken by force from 

 the mantle. 



The number of notches in the plates of insertion is sometimes, but 

 very rarely, liable to variation. In one specimen of Chiton Bowenii 

 in the Museum Collection, the plate of insertion of the last valve but 

 one has two notches on one side, but the normal single notch of the 

 genus on the other. 



" A Chiton has a carapace like an isopod Crustacean, a dorsal vessel 

 like an Annelid, bilateral symmetrical reproductive viscera like an 

 acephalous Mollusk, a head and foot like a patelloid Gasteropod, a 

 posterior anus like the Fissurellidce, and branchiae like those of the 

 brachyurous Crustacea ! Such manifold affinities at once unite and 

 sever this odd group from several most dissimilar classes." — Br. T. 

 Williams, Ann. fy Mag. Nat. Hist. xvi. 408 (1855). 



