260 COMPARATIVE ZOOLOGY. 
pallial sinus; two muscular impressions: as “the giant of 
the bivalve race” ( 7’r¢daene) and Cockle (Cardium)."” 
4. Mantle, with three openings; siphons large; pallial 
sinus and muscular impressions well marked: as the com- 
mon Clam (Venus), and the burrowing Razor - shell 
(Solen). 
Crass V.— Gasteropoda. 
The Snails are, with rare exceptions, all univalves.’® 
The body is coiled up in a conical shell, which is usually 
spiral, the whorls passing obliquely (and generally from 
right to left)” around a central axis, or “columella.” 
When the columella is hollow (perforated), the end is 
called the “umbilicus.” When the whorls are coiled 
around the axis in the same plane, we have a discoidal 
Fia. 219.—Whelk (Buccinwm), showing operculum, o, and siphon, 8. 
shell, as the Planorbis. The mouth, or “aperture,” of 
the shell is “entire” in most vegetable-feeding Snails, and 
notched or produced into a canal for the siphons in the 
carnivorous species. The former are generally land and 
fresh - water forms, and the latter all marine. In some 
Gasteropods, as the River-snails, a horny or caleareous 
