PELECYPODS 417 
firmly together and are placed one upon either side of the animal. 
The two valves are always held together tightly along a dorsal 
margin by a “hinge ligament,” an exceedingly tough, leathery 
substance, and they are usually secured the more firmly by a 
system of interlocking teeth, which project from the hinge. The 
opening and shutting of the valves is controlled by the adductor 
muscles of the animal, by the 
ligament which binds the valves 
together, and by the cartilage, an 
elastic pad of rubber-like appear- 
ance which is lodged just with- 
in the hinge, and is compressed 
when the valves are closed to- 
gether. Thus, when the valves 
are closed, there is always a strain 
upon the adductor to overcome 
the elastic resistance of the car- 
. 7 The interior characteristics of a pelecypod 
tilage. When a pelecypod dies shen. rett valve of Venus gnidia: 4, anterior, B, 
posterior, C, dorsal, D, ventral margin; 4B, 
and the pull of the adductor mus- length, CD, breadth of. shell; a.m., anterior, 
7 p.m., posterior impressions of adductor muscle; 
cles 18 released, the valves gape p, pallial line; p.s., pallial sinus; 2, ligament; 
open. lw., lunule; wu, umbo; c, cardinal teeth; a.J., an- 
terior lateral tooth ; p. l., posterior lateral tooth. 
The summit or apex of the 
valve is called the wmbo, or beak. It is usually twisted to some 
extent, and in certain forms develops a suggestive spiral figure. 
The beaks of the valves are the points where the shell-growth 
begins, the secretion of calcareous matter being made by the man- 
tle margin and deposited about the edges of the valves; the 
growth is therefore constantly away from the umbones. 
The umbones usually point forward. In many species they 
touch each other over the hinge-margin or approximate very 
closely ; in other species the hinge-margin is very wide and the 
beaks are comparatively far apart. The hinge-margin, lying adja- _ 
cent to the umbones, is known as the dorsal margin of the shell, in 
contradistinetion to the ventral margin, opposite the umbones. 
The anterior margin is the front edge and the posterior margin the 
hinder edge of the shells, through which the siphons may project. 
When the umbones are about central in respect to the posterior 
27 
