BIVALVES. 



411 



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 

 a thick, often glossy periostracum, varying in colour, the prevailing tints being 



large iuver-mussel, Anodonta cygnea (nat. size). 



greenish olive, brownish yellow, brown, and black. Many are beautifully rayed 

 with green. The solidity of many of the species — especially the North American 

 forms — is remarkable, although others are thin and fragile. Some are pearly 

 within, and others white, pinkish purple, salmon -colour, yellow, or iridescent. 

 The shells are marked with two adductor scars, and the pedal scars are also often 

 clearly visible, while the pallial line is uninterrupted by a posterior sinus. The 

 animal of Unio has the lobes of the mantle free, excepting posteriorly, where 

 they are connected, forming two orifices, the lower or branchial for the passage of 

 the water to the gills, and the upper for excretal purposes. The former is 

 fringed with several rows of papillae. The foot is large, thick, tongue-shaped, 

 and used as a creeping and burrowing organ. The sexes are united in the 

 European species, but distinct in the American. A remarkable feature in con- 



