MOLLUSCA. 259 
rocks by a cord of threads called “byssus ;” but the rest 
have a “foot,” by which they creep about. Unlike the 
Oyster, also, the majority live in an erect position, resting 
Fig. 217.—Lamellibranch (Mactra): a, foot; 6, ¢, siphons. 
on the edges of their shells. Over 4000 living species are 
known. These are fresh-water and marine, and range 
from the shore to a depth of a thousand feet. 
The chief characters for distinguishing Lamellibranchs 
are the muscular impressions,’ whether one or two; the 
presence of a pallial sinus, which indicates the possession 
of siphons; the structure of the hinge; and the symmetry 
of the valves. 
The following are the leading types of structure: 
1. Mantle open; no siphon-tubes, and therefore no pal- 
lial sinus; one muscular impression; foot wanting, or very 
small; shell unequivalve and edentulous: as the Oyster 
(Ostrea), Scallop (Pecten), and Pearl 
Oyster (Avicula).'"” 
2. Mantle open in front,and closed 
behind, except one small aperture ; 
no pallial sinus; two muscular im- 
pressions; foot large; shell equi- 
valve: as the Sea Mussel (M/yéz/us), 
and Fresh-water Clam ( W/nzo). 
3. Mantle having three openings; Fre. 215.—Cockle (Curcivin cos- 
tatum); one-third natural 
tubes wanting, or very short; no © size, China seas. 
