2T2 



COMPARATIVE ZOOLOGY. 



tiilous— as the Oj-ster {Ostrea), Scallop {Pecten), and Pearl 

 Oyster {Avicula). 



2. Heteromya: with two unequal adductor rftuscles 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 

 ^t^^':T^'ZZ^ gia"t of the bivalve race" {Tridac- 

 size. China eeas. nd). h. Thosc with siphons and pal- 



lial sinus — as the common Clam {Mya), Quohog {Venus), 

 and Eazor-shell {Soleny*" 



Class II. — Gasteropoda. 



The Snails are, M'ith rare exceptions, all univalves."' 

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



Fio. 229.— Wlieik (Buceimim), showing opeicnliim, o, mid siphon, ». 



spiral, the whorls passing obliquely (and generally from 

 riglit to ■'left),'" around a central axis, or " columella." 



