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 
( Unio), Cockle ( Cardium ,), and “ the 
’ZZ «< the M«I« race” (TriUc 
size, china seas. na ) % 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 
Fig. 228.—Whelk ( Buccinum ), showing operculum, o, and siphon, 8. 
spiral, the whorls passing obliquely (and generally from 
right to left)," 2 around a central axis, or “ columella.’’ 
