82 Cincintiati Society of Natural History. 



to the margin. Beak high, pointed and projecting beyond the 

 cardinal margin a little forward of the middle of the hinge 

 line. There are two costse on the po.sterior ear, and three or 

 more finer ones on the anterior ear. There are about twelve 

 principal radiating ribs on the body of the shell and about 

 half as many rudimentary and intercalated shorter ribs between 

 them ; they are separated by wider flattened spaces. It is dif- 

 ficult to tell from our specimens whether or not there were 

 any concentric lines, though they are quite well preserved ; 

 if there were such lines they were very fine. 



This species approaches more nearly to A. rectilaioarius 

 than to any other, but it may be readily distinguished by the 

 scarcity of the radiating ribs and the wide flattened spaces 

 between them ; the beak, too, is higher and more pointed, as 

 well as having a more tumid umbo. 



Found in the Coal Measures on Elkhorn Creek, in Kentucky, 

 and now in the collection of Mr. Falser. 



Oktiiodesm.v mundum, n. sp. 



Plate /, /'Vi,'. //, left valve slightly brokeji at the fostcrior 

 Old ; Fig. J 2, cardinal vietv, natural size. 



Species a little below the medium size. Shell elongate, more 

 than twice as long as wide, thickness less than half the width. 

 Cardinal and ])asal lines diverging at a slight angle posteriorly. 

 Anterior end forward of the beaks very thin, contracted 

 beneath the beaks, then somewhat extended and abruptly 

 rounded to the basal margin. Posterior end broad, obliquely 

 rounded and produced at the postero-ba.sal angle, which is 

 abruptly rounded into the base. Beaks anterior, small and 

 produced beyond the cardinal line. Umbones compressed 

 and flattened toward the basal margin and forming a ridge on 

 each valve directed toward the postero-basal angle, above 

 which they are depressed into a thin, wing-like expansion 

 toward the postero - cardinal border. vSurface marked with 

 fine concentric lines. 



This species can not be readily confounded with any other 

 described form, and no comparison is, therefore, necessary. 



Found in the Hudson River (iroup, near the top of the hills, 

 at Cincinnati, and now in llie collection of Mr. h'aber. 



