20 



margin straight or very slightly convex in the middle, and strongly 

 rounded upward toward each extremity. Body of the shell often dis- 

 tinctl y angular along the posterior-umbonal ridge and the postero-cardi- 

 nal slope quite abrupt ; some individuals, however, are obtusely rounded 

 on the umbonal ridge. Surface of the shell marked by rather strong 

 concentric ridges of growth, which are crossed, on the rounded anterior 

 end and middle of the-valve, by extremely flue radiating striae, only 

 detected near the basal portion of the larger individuals by the aid of a 

 moderate magnifying power. 



There may be some little doubt as to the true generic relations of this 

 shell, but it possesses so many of the features of Trapezium that we have 

 not hesitated to refer it to that genus ; the almost subceutral and some- 

 what prominent beak forms the strongest point of difference. It differs 

 in this respect from th e preceding species, as also in the general form 

 of the valves, and in the surface markings. The shells of the two 

 species have nearly the same dimensions, and in their mutilated and 

 imperfect condition, as seen imbedded in the rock, may readily be con- 

 founded, but the more prominent beaks and longer anterior end of this 

 one will at once distinguish them. 



Formation and locality. — In a hard white sandstone of Jurassic age, 

 associated with Pseudmonotis curta just above the Triassic red beds, at 

 Eed Caiion Creek, Black Hills, Dakota. 



Family uncertain. 



Genus PLEUROMYA, Agassiz. 



Pleueomya Newtoni. 



Plate 5, figs. 19 and 20. 



Shell of moderate size, transversely subovate or triangularly ovate, 

 the greatest height being across the valve nearly on a line with the 

 beaks. Valves strongly convex near the anterior end, and in a cardi- 

 nal view acutely wedge-shaped, thickest opposite the beaks and rapidly 

 decreasing posteriorly. Beaks large, strong, enrolled, and approxi- 

 mate, situated at, or within, the anterior third of the shell and project- 

 ing largely above the 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 distinctly flattened. Posterior end 



• The family relations of this shell, as well as some others herein described, are not 

 yet satisfactorily determined, and we do not like to refer them to a family to which 

 we feel certain, from their characters, as far as known, they have no resemblance or 

 affinity; we rather prefer to leave them in doubt, and as subjects for future investiga- 

 tion. The reference of Phuromya and MyaeUea to the same family.with AnaUna 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 struotnial features, and 

 these we deem the ny sure and reliable means of classification. 



