272 



COMPARATIVE ZOOLOGY. 



tnlous as the Oyster (Ostrea), Scallop (Pecteri), 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 

 ( Unw\ Cockle (Cardium), and " the 

 S iant f the bivalve race" (Tridac- 

 ne ^ . Those with siphons and pal- 

 lial sinus as the common Clam (Mya), Quohog ( Venus), 

 and Razor-shell (tSoleri). 1 *" 



CLASS II. Gasteropoda. 



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

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



Pio. 228. Whelk (Buccinum), showing opercnlum, o, and siphon, . 



spiral, the whorls passing obliquely (and generally from 

 right to left), 1 " around a central axis, or "columella." 



