34 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 U. nasatus, Say, may be considered the type. Its form and appearance is 
altogether graceful. It perhaps more closely resembles Fisherianus, (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 Stewarpsonu. Pl. XXIII. Fig. 36. 
Testa levi, triangulari, compressa, inequilaterali, postice emarginatd, ad basim prelonga, ad lateré planu- 
lata; valvulis crassis ; natibus prominulis; epidermide luteo-fuscd, radiatd ; dentibus cardinalibus subcrassis, 
crenulatis ; lateralibus brevissimis, crassis, crenulatis rectisque; margarita argented et iridescente. 
Shell smooth, triangular, compressed, inequilateral, emarginate behind, prolonged at the base and flattened at 
the side; valves thick; beaks 1ather 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 
Unio foliatus, Hild., 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, 
