118 



Unio subtentus.— Oblong-oval, subcompressed, slightly con- 

 tracted at the 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 

 suhglobosa, nobis, and Fusus fluvialis, 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. — Shell 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, 182G.] 



Delphinula ? LAXA. — 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. 



