134 



before, but gradually diminishing (as that of some Alasniodontas ;) 

 lateral teeth rather slender. 



Length (at the anterior tip of the lateral teeth) one inch and 

 one-fifth ; 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 Chalco ? 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, subrostrated, 

 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 the 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. Tlie following notice, 

 however, I have concluded to publish, because it relates to the most 

 common species in our eastern waters. 



