272 



COMPARATIVE ZOOLOGY. 



tulons — 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 

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

 giant of the bivalve race" (Tridac- 

 ne). b. Those with siphons and pal- 

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

 and Eazor-shell (Solen). 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 



Fig. 227. — Cockle (Cardium cos- 

 tutum) ; one third natural 

 size. China seas. 



Fig. 228.— Whelk (Buccinum), showing operculum, o, and siphon, 8. 



spiral, the whorls passing obliquely (and generally from 

 right to left), 142 around a central axis, or "columella." 



