272 
COMPARATIVE ZOOLOGY. 
tulous—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 
( TJnio), Cockle ( Oardium ), and “ the 
F 2Z?TJTrfZZ gi“"t of the bivalve race” {Tridac- 
size. China seas. J. Those with siphons and pal¬ 
lial sinus—as the common Clam {Mya), Quohog ( Venus), 
and Kazor-shell {Solen)A* 
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 siphon, «. 
spiral, the whorls passing obliquely (and generally from 
right to left), 143 around a central axis, or “columella.” 
