272 



COMPARATIVE ZOOLOGY. 



tiilous as the Oyster (Ostrea\ Scallop (Pecten\ and Pearl 

 Oyster (Avicula). 



2. Heteromya: with two unequal adductor muscles and 

 no siphons as the Sea-mussel (My- 

 tilus). 



3. Isomya : with two equal ad- 

 ductor muscles. There are two sec- 

 tions of this order: a. Those with 

 no siphons, and hence no pallial 

 sinus as the Fresh -water Mussel 

 ( Unio), Cockle (Cardium), and " the 

 g^nt of the bivalve race" (Tridae- 

 size, china seas. ne ). J. Those with siphons and pal- 



lial sinus as the common Clam (My a), Quohog ( Venus), 

 and Razor-shell (Solen). 140 



CLASS II. Gasteropoda. 



The Snails are, with rare exceptions, all univalves. 141 

 The body is coiled up in a conical shell, which is usually 



Fio. 228. Whelk (Buccinum), showing operculum, o, and siphou, . 



spiral, the whorls passing obliquely (and generally from 

 right to left), 148 around a central axis, or "columella." 



