32 THE SHELL-FISH OF THE COAST. 
shell (Crucibulum striatum, Pl. 1, Fig. 32), which is, 
however, rarely seen with us. 
No true limpets, which are rock-loving animals, 
are known to inhabit the New Jersey coast. 
CLAMS AND THEIR ALLIES (BIVALVES). 
We call these shells ‘ bivalves’ because they are 
each made up of two pieces or valves, which lie on 
either side of the animal, and are respectively desig- 
nated the ‘right’ and the ‘left’ valve. But how 
do we determine which is which? Barring the 
case of the oysters, scallops, and a few of their 
friends, the bivalves or headless mollusks have the 
valves of the shell almost invariably equal, and, 
with insignificant exceptions, the beaks of the 
valves, known to systematists as the ‘umbones’ 
(singular, ‘umbo’), are directed forward. Bearing 
this fact in mind,—ie., knowing which is front 
and which back,—it is an easy matter to determine 
the two sides. Possibly you may have stumbled 
across one of the hard-shell clams from which the 
animal has been dislodged, but which still holds 
both valves together. The valves in this case will 
be wide open, and are pulled and held in this posi- 
tion by an elastic ligament which runs along the 
back of the shell. Look on the interior surfaces 
of the valves, and you will observe, both in front 
and in the rear, a nearly round, impressed scar, the 
positions of which correspond in the two valves. 
Uniting the scars of the opposite valves, there were 
in the living condition of the animal two stout mus- 
cular bundles, whose contraction, regulated by the 
