278 DESCRIPTION OF NEW 



straight, and thickened at the posterior end; anterior cicatrices distinct; posterior cica- 

 trices confluent; dorsal cicatrices placed nearly in the centre of the cavity of the beaks; 

 palleal cicatrix impressed ; cavity of the shell very shallow ; cavity of the beaks shallow 

 and rounded ; nacre purple and iridescent. 



Remarks. — I have before me two specimens of this species, which belongs to the group 

 of which the I), nasatus, Say, may be considered the type. Its form and appearance is 

 altogether graceful. It perhaps more closely resembles Fislwrianus, (Nobis,) than any 

 other species, but may be distinguished in being rather less wide, and it differs in having 

 broader rays. In outline, it resembles U. Barrattianus, (Nobis,) but it is a wider shell, 

 and not so thick. The two specimens I have are both of a fine purple ; others may differ, 

 and it may be found white and salmon colour. 



Unio Stewardsonii. PI. XXIII. Fig. 36. 



Testa Ixvi, triangulari, compressd, insequilaterali, postice emarginatd, ad basim prmlongd, ad latere ptanu- 

 lald; valvulis crassis; natibus prominidis ; epidermide luteo-fuscd, radiatd ; dentibus cardinalibus subcrassis, 

 crenulatis ; lateralibus brevissimis, crassis, crenulatis reclisquc ; margaritd argented et iridescenle. 



Shell smooth, triangular, compressed, inequilateral, emarginate behind, prolonged at the base and flattened at 

 the side; valves thick; beaks lather prominent; epidermis yellowish-brown, radiated; cardinal teeth rather thick, 

 crenulate; lateral teeth very short, thick and straight; nacre silvery white and iridescent. 



Hab. Chatanooga river, Tenn., Thomas Stewardson, M. D. 

 Cabinet of the Academy of Natural Sciences. 

 Diam. .6, Length 1, Breadth 1.2 inches. 



Shell smooth, triangular, compressed, inequilateral, emarginate behind, enlarged and 

 extended circularly at the base, flattened at the side; substance of the shell thick, thinner 

 behind; beaks rather prominent; ligament short and rather thin; epidermis yellowish 

 dull-brown, with numerous capillary rays over most of the disk, and with distant marks 

 of growth; posterior slope flattened and lighter in colour; umbonial slope obtusely angular; 

 cardinal teeth rather thick, crenulate, double in both valves; lateral teeth very short, cre- 

 nulate, thick, nearly straight, and thickened at the posterior end; anterior cicatrices dis- 

 tinct; posterior cicatrices distinct; dorsal cicatrices placed near to the margin and almost 

 in connexion with the cardinal tooth; palleal cicatrix impressed; cavity of the shell very 

 shallow; cavity of the beaks shallow and subangular; nacre silvery white and iridescent. 



Remarks. — Among many interesting specimens of some twenty species, brought by 

 Dr. Stewardson, was a single specimen of this species. It is very closely allied to the 

 Uniofoliatus, Mild., and at first I did not hesitate to consider it as a small specimen of 

 the female of that species, but a very slight examination of the enlargement of that portion 

 of the shell which indicates the female character, satisfied me that it could not be the 

 same. In the foliatus the enlargement for the oviducts is always a continuation and 

 enlargement of the ridge over the middle of the disk; and immediately before this there 

 is, in the mature female, a deep indentation. In the Stewardsonii this emarginate part is 

 replaced by the enlargement, and instead of being indented at the posterior basal margin, 



