MOLLUSCA. 257 
line let fall from the hinge divides the shell into two 
equal parts. The ventral valve has, in the great major- 
ity, a prominent beak, perforated by a foramen, or hole, 
through which a fleshy foot protrudes to attach the ani- 
mal to submarine rocks. The valves are opened and shut 
by means of muscles, and in some cases they are hinged, 
having teeth and _ sockets 
near the beak. The mouth 
faces the middle of the 
ft 
f fi oy | 
cD 
il iM 
Fie. 213.—A Brachiopod (Terebratulina 
septentrionalis). Atlantic coast. 
margin opposite the beak ; Fig. 214. Boe Valve of a Brachiopod 
and on either side of it is (Terebratula), showing, in descending 
b a order, cardinal process, dental sockets, 
a long fringed “arm,” gen- hinge-plate, septum, and loop support- 
= ing the ciliated arms. 
erally coiled up, and sup- Fi 
y ) 
ported by a bony frame-work. The animal, having no 
gills, respires either by the arms or the mantle. Brachi- 
opods were once very abundant, over 2000 extinct species 
having been described; but less than a hundred species 
are now living.“ They are all marine, and fixed; but of 
all Mollusks, they enjoy the greatest range of climate and 
depth. 
Crass I1V.—Lamellibranchiata. 
Lamellibranchs are all ordinary bivalves, as the Oyster 
and Clam. The shells differ from those of Brachiopods 
in being placed on the right and left sides of the body, so 
that the hinge is on the back of the animal, and in being 
anequilateral and equivalved.’” The umbo answers to the 
ay 
