32 THE SHELI^FISH OF THE COAST. 



shell {Orucibulum striatum., PI. 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, aud, 

 with insignificant exceptions, the beaks of the 

 valves, known to systematists as the ' umbones' 

 (singular, ' umbo'), are directed forward. Bearing 

 this fact in mind, — i.e., knowing which is front 

 and which back, — it is an easy matterto 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-r 

 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 



