118 
Unio subtentus.— O blong-oval, subcompressed, slightly con- 
tracted at tbe middle of the base, dull yellowish brown ; beaks not 
prominent, decorticated ; dorsal edge regularly arcuated, without 
any appearance of an angle ; anterior margin with numerous slightly 
reflected ribs. 
Breadth three and one-fifth inches ; length more than one and 
a half inches. 
This species was found by Professar Vanuxem in the north fork 
of the Holstein River, in company with U. cariosus^ nobis, Melania 
Hubglohosa, nobis, and Fusus Jiuvialis, nobis. 
In consequence of the ribbed appearance of the anterior margin 
of the shell, it has much similarity to an Alasmodon^ but the 
lamelliform teeth are prominent and distinct. 
Alasmodonta ambigua. — S hell transversely oblong-oval ; 
somewhat inflated, rather thin, dusky, wrinkled ; anterior and 
posterior margins almost equally rounded ; hinge margin parallel 
with the base ; basal margin a little compressed in the middle ; 
beaks not prominent, approximate, their surface slightly undulated ; 
a very obtuse, hardly elevated undulation extends from the 
beaks to the junction of the basal and anterior margins ,* hinge 
with very small, obsolete primary teeth; within somewhat iri- 
descent ; posterior and basal portion milk-white ; hinge cavity 
brownish. 
Breadth one and four-fifth inches ; length four-fifths nearly. 
This is one of the many fine shells which I obtained in the 
North-western territory, when travelling with Major Long’s party. 
It forms a link between the genera Alasmodonta and Anodonta. 
When young the primary teeth are obvious, but when the shell 
arrives at the full growth the teeth are obsolete, and in some in- 
stances, not at all visible. 
[J. A. N. S. vol. V. p. 207, et seqq., Feb. 1826.] 
Delphinula ? LAXii.— Whorls all separate. 
Inhabits the coast of South Carolina. 
Shell subovate, nearly glabrous, whitish, slightly tinged with 
dull yellowish ; whorls rounded, perfectly disjoined throughout, 
rapidly attenuated to the apex ; a dilated, rugous, shallow groove 
on the line of the umbilicus ; aperture oval-orbicular, with an acute 
edge. 
