THE BIVALVES 



bivalves. The family ALtheriicUe includes a few remarkable bivalves known as fresh- 

 water oysters. They occur in the Nile and some other rivers of North Africa, and 

 some parts of South America. When young, sEtheria is a freely creeping mollusk, 

 but when adult becomes attached to stones and other substances like the oyster. 

 The shells are irregular in their growth, and are of an olive-green color. The some- 

 what pearly interior of the valves is marked with two adductor scars, and the pallial 

 line is entire. They may be regarded as irregular forms of Unionidce without a 

 foot, modified for a sedentary life. 



The numerous kinds of fresh-water mussels (Unionidce and Mutelidce} occur 

 in the lakes and rivers of all continents, and the large islands of the Malay 



LARGE RIVER MUSSEL, Anodonta cygnea. 

 (Natural size.) 



Archipelago and New Zealand; although in most of the smaller islands they 

 are unknown. North America, and especially the drainage area of the Mississippi 

 and its tributaries, is the great home of the Unionidce. The species may 

 be counted by hundreds, some being the most remarkable and beautiful that 

 exist in any part of the globe. The shells are usually equi valve, and joined by 

 an external ligament, but exhibit great variation in shape. The hinge is some- 

 times destitute of teeth (Anodonta, Mycetopus); or powerfully formed with 

 strong complex interlocking teeth, as in- many of the North-American forms; or 

 it may consist of very numerous teeth on a straight hinge line (Pliodon),. 

 recalling the form of hinge obtaining in the Arcidce. The exterior is covered with 



