served to differ very essentially from that common species. The 
umbo and beaks of the pUcatus are very prominent and altogether 
destitute of any small literations or divaricating lines^ having only 
the origin of one or two of the large undulations grooved upon it. 
The primary teeth of the plicatus also are much thicker than those 
of this species. 
I formerly considered this species, with much doubt, as distinct 
from the undulatus of Barnes, and gave to it the name of heros^ 
but notwithstanding some differences, I have concluded, after a 
more mature examination and comparison, that it may be with 
propriety referred to that species. Barnes drew his description 
and figure from a specimen then unique, belonging to the cabinet 
of Mr. S. B. Collins, which was so eroded as not to exhibit the 
ornamental tubercles of the umbo and beak. 
The dimensions of a fine specimen found by Mr. 0. Evans are, 
length five inches, breadth over seven, and convexity two inches 
and seven-tenths. 
It is not uncommon in Eox river, an arm of the Wabash, in 
muddy and sluggish parts of the stream. PL 16. 
Unio abruptus. — Specific character. Subquadrate, truncate 
before. 
Desc. Shell oval-quadrate, yellowish olive or brownish, slightly 
radiated with blackish-brown narrow lines, which do not extend to 
the base : umbo not very prominent, rounded, slightly compressed, 
generally radiated with obscure greenish, and placed far back, 
almost terminal : disk convex, anteriorly with a hardly raised, very 
obtuse elevation, becoming obvious towards the margin and a little 
prominent at the anterior basal angle : hinge margin a little de- 
pressed, so as to form an angle with the disk, more acute towards 
the umbo : ligament higher than the beaks : anterior margin trun- 
cated by an almost straight line : basal margin very slightly con- 
tracted : posterior margin very short, gaping considerably from the 
middle of the base to the cavity of the hinge membranes : cavity 
of the hinge membranes very obvious, rather wide, as long or 
longer behind the beaks than broad : within white, or tinged with 
very fine fulvous-pink, or salmon color ; posterior cicatrix profound ; 
smaller cicatrix rather large, oblique, arquated : palleal impression 
hardly more prominent than the anterior cicatrix : cardinal teeth 
thick, direct ; that of the left valve subtrifid ; tooth plate rather 
