68 ; ZOOLOGY, 
ploughing furrows in their course, the ventral half of the shell being generally 
sunk in the bottom, which gives it a vertical position. 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 pl. 76, fig. 38, where we have observed it in the 
Ohio, does not take the more or less horizontal position of those like 
pl. 16, 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. 7 
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 rivers 
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 spinosus, 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 pl. 76, fig. 47) are 
so great that not less than fifty specimens are required to illustrate it 
properly. 
' The Unionidee 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 River, 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 
