JUKASdlC FOSSILS. 367 



Family uncertain.* 



Genus PLEUROMYA Agassiz. 



PLEUllOMYA NEWTONI. 



Plato r>, figs. 19, 20. 



Pleuromya newtoni Whitf., Prelim. Kept. Pal. Black Hills, 1877, p. 20. 



Shell of moderate size, transversely subovate or triangularly ovate, the 

 greatest height being across the valve nearl}^ on a line with the beaks. 

 Valves strongly convex near the anterior end, and in a cardinal view acutely 

 wedge-shaped, thickest opposite the beaks and rapidly decreasing posteriori)'. 

 Beaks large, strong, enrolled, and approximate, situated at or within the 

 anterior third of the length of the shell, and projecting largely above the 

 cardinal line. Hinge line not exceeding one-half the length of the shell, 

 distinctly arched ; anterior end obliquely subtruncate, longest below at its 

 junction with the basal margin, and in some cases the anterior slope is dis- 

 tinctly flattened. Posterior end narrowed, the postero-cardinal margin 

 rapidly declining from the extremity of the hinge line to the longest point 

 of the valve, which is narrowly rounded and at about the middle of the 

 height; basal margin very slightly constricted or emarginate, just within 

 the anterior third of the length, by a faint sulcus, which passes from the 

 surface of the umbo to the base of the shell; posterior to this it is strongly 

 convex, and as it approaches the posterior third of the length is more 

 strongly rounded upward to the extremity. Body of the valve convex, 

 most ventricose on the umbones and along the middle of the valve; ante- 

 rior umbonal ridge obtusely angular; anterior slope abrupt and sometimes 

 flattened obliquely; posterior umbonal ridge scarcely defined or broadly 

 rounded. Immediately behind the anterior umbonal ridge a broad unde- 

 fined sulcus passes across the valve, slightly modif^-ing the direction of the 



*The family relations of this shell, as well as some others herein described, are not yet satis- 

 factorily deteruiiuc'd; and we do not like to refer them to a family to which we feel certain, from their 

 eharacters as far as known, they have no resemblance or affinity; we rather prefer to leave them in 

 doubt, and as subjects for future investigation. The reference of Pleuromya and Myacitcs to the same 

 family with Anatina, we deem unnatural, as from the character of the shell and the consequent habits 

 of the animal they must have been very far removed from each other in structiual features, and these 

 we deem the only sure and reliable means of classification. 



