68 ZOOLOGY. 



ploughing furrows in their course, the ventral half of the shell being generally 

 sunk in the bottom, which gives it a vertical posifion. Some inhabit gravel 

 bars ; and as it is difficult for them to progress through such an unyielding 

 material, these species seldom change their place. A few species, as the 

 genus Mycetopus, live in perforations made apparently with the foot, which 

 has a peculiar development. These species of Unio, when left dry by the 

 fall of the rivers, bury themselves in the moist sand. Unio (Truncilla) 

 triqueter, a shell much like^Z. 76, fig. 38, where we have observed it in the 

 Ohio, does not take the more or less horizontal position of those like 

 pi. T6, figs. 21, 47, but the anterior extremity is sunk diagonally into the 

 mud or gravel, so as to bring the posterior truncation level with the 

 bottom. 



The shell is composed of nacre, and varies from little more than the 

 thickness of paper to half an inch, or more, according to the species. It is 

 covered with a periostraca, usually colored with various tints of yellow, 

 green, or brown, sometimes ornamented with markings or radiating lines 

 of a different color, among which green upon a yellowish ground is the 

 most frequent. The surface is plain, as in the species inhabiting the rivere 

 of the Atlantic coast of the United States, or with undulations or knotty 

 projections, as in those of the tributaries of the Mississippi. Two species, 

 Unio spinosns^ Lea, from Georgia, and U. collinus, Conrad, from Virginia, 

 have a few spines upon the shell. The beaks, being the oldest part, and 

 the most exposed to the action of abrading agents, are subject to be worn 

 off, and to such an extent that the younger portion in some cases disappears, 

 and is replaced by successive secretions of shelly matter from the mantle. 

 This renders it difficult to refer young and old shells to the same species. 

 On this account no cabinet can be deemed complete without series 

 indicating the gradual changes and variations in size, and other charac- 

 teristics to which individual species are subject. 



The variation in external character is extremely great in this family, 

 where the same species will, in some instances, vary more than distinct 

 species, which present more uniform characteristics. The variations of 

 Unio complanatus (one of which is well represented in pi. 76, fig. 47) are 

 so great that not less than fifty specimens are required to illustrate it 

 properly. 



The Unionidse inhabit the rivers and ponds in Europe, Asia, Africa, 

 Australia, both Americas, and particularly those of the United States. 

 Their distribution here has not been as thoroughly studied as it deserves to 

 be, but the following facts may be stated. The species inhabiting the 

 rivers of the Atlantic coast (excepting U. viridis, which is found in 

 Kentucky, and U. cariosus, which has been taken in White Kiver, Indiana) 

 do not occur in the tributaries of the Mississippi. A few western species, 

 as U. alatus, siliquoideus, ventricosus, and pressus, have found their way 

 through New York, as far east as Lake Champlain. U. rectus is found on 

 the southern border of Lake Superior, in Lake Champlain, the Ohio and 

 Alabama rivers, presenting a wide range. U. complanatus, although it 

 seems not to be found in the tributaries of the Mississippi, occurs in some 

 272 



