FOSSILS OF THE JUEASSIC PEKIOD. 291 



obtuse at the outer angle, the posterior margin sloping gently backward 

 toward the body of the shell, the posterior wing not fully determined. Body 

 of the (n'ght 1) valve convex, most prominent, and almost subangular toward 

 the umbo; beak small, pointed, scarcely projecting beyond the hinge-line; 

 margin of the shell broadly rounded anteriorly and posteriorly, and some- 

 what acutely rounded at the base. 



Surface of the valve in the specimen seen marked by fine, concentric, 

 impressed lines, and by stronger radiating lines. These latter seem to be 

 composed of rows of fine dots, or punctures, and are strongly divergent, so 

 as to curve abruptly upward toward the margin on the sides of the valve, 

 the increase being entirely by interstitial addition. 



The shell differs from C. heUistriata Meek in its smaller size, convex 

 valves, more erect form, and in the proportional height and width of the 

 valve, this being higher than wide, while the reverse is true of the other. 



There can be no doubt that this is the shell to which Mr. Meek gave 

 the above names, although he had not seen the radiating strise of the 

 surface so plainly as to feel sure of their existence. On the specimen used 

 in the above description, the strise are very distinct, and have exactly the 

 character of that of C. heUistriata; consequently, there can be no doubt of 

 its correct reference to the genus Camptonectes, although in our specimens 

 the anterior wing and sinus have not been satisfactorily determined, that 

 part in the figure having to be made from analogy. 



formation and locality. — In light-colored shales of Jurassic age, north- 

 west of Rawlings Station, Wyoming. 



Camptonectes pertenuistriatus n. sp. 



Plate VII, fig. 17. 



Shell small, very broadly ovate, the point of greatest width being about 

 the middle of the length, width equal to about four-fifths of the height; 

 base and sides of the shell regularly rounded. Right valve very depressed- 

 convex or almost flattened; hinge-line short, the anterior side equal to a 

 little more than one-third of the width of the valve below; posterior side 

 unknown, the specimen being mutilated in this part. Beak small, pointed, 

 not rising above the line of the hinge; cardinal slopes nearly straight from 



