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 
( Unio), Cockle ( Cardium ), and “ the 
F, ^)T C r u e le ttofr,S S iant of tlie bivalve race” (Tridae- 
size, china seas. na y Those with siphons and pal¬ 
lial sinus—as the common Clam ( Mya ), 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 siphon, «. 
spiral, the whorls passing obliquely (and generally from 
right to left), 142 around a central axis, or “ columella.” 
