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before, but gradually diminisbing (as that of some Alasmodontas ;) 
lateral teetb. ratber slender. 
Length (at tbe anterior tip of tbe lateral teetb) one inch and 
one-fiftb ; breadth, nearly two inches and one-fifth. Convexity, 
four-fifths of an inch. Inhabits Mexico. 
This shell is remarkable for the coppery tint, which is more 
obvious on the umbones. It was obtained in Lake Chaleo ? and 
was presented to me by Mr. Maclure. In the young state it is 
radiate with green before, and the adult has a very thick granular 
deposite of carbonate of lime on the anterior half of the shell, 
which is in the water and above the mud. The species appears to 
be abundant. 
Unio subrostkatus.— Transversely elongated, sub rostrated, 
radiated. 
Inhabits Wabash. 
Transversely elongate subovate, brownish or pale ochreous, with 
numerous dark green radii : beaks but little elevated with a few, 
small angular, concentric lines : ligament margin a little compress- 
ed : anterior margin somewhat elongated, hardly subrostrated : 
separated by an obtuse angle from the ligament margin : base 
arquated : posterior margin rounded : within white, often slightly 
tinted with flesh color; somewhat iridescent on the margins, 
particularly the anterior margin : substance of the shell not thick : 
teeth very oblique, crested. 
Length, one inch and two-fifths ; breadth, three inches and one- 
fifth. Convexity, hardly over one inch. 
This may be said to be the analogue of the U. nasutus, nob., 
of the Western waters. The rostrum, however, is not so definite, 
and it is a more convex shell. The aged shell is dark brown, but 
near the beaks reddish brown. 
Note. — A short time since, I began a review of tbe American species 
of Unio hitherto described by naturalists, in which it appeared to me 
that much yet remains to be done, notwithstanding the laudable attempts 
that have already been made to rectify the synonyma : but on advancing 
somewhat in the undertaking, I found myself so much at a loss for 
books and other means of reference in my insulated residence, that I 
was compelled for the present to relinquish it. The following notice? 
however, I have concluded to publish, because it relates to the most 
common species in our eastern waters. 
