622 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



almost vertically subtruncate; ventral margin broadly rounded; surface 

 sculpture unknown. There is a very obscure posterior umbonal ridge, and 

 the muscular and pallial impressions are not clearly shown on the cast. 



One cast showing impression of part of hinge has three strong cardinal 

 teeth, of which the posterior one is very long and oblique. The specimen 

 is not in condition to show whether lateral teeth are present. 



Height of an averag-e shell, 28 mm.; length, 32 mm.; convexity of two 

 valves united, 15 mm. The largest specimens in the collection have the 

 corresponding dimensions about one-fifth greater. 



The only described American Jurassic species with which this need be 

 compared is Dosinia jurassica Whitfield, which is a smaller, more convex 

 species, with less prominent beaks and slight differences in outline. It is 

 not probable that the two species are closely related. 



Collected from Cinnabar Mountain; divide between Fawn Creek and 

 Gallatin Valley; east end of northeast spur from Signal Peak; saddle in 

 ridge west of south head of Gardiner River; head of Fawn Creek northeast 

 of Monument Peak, and ridge between Basin and Red creeks, near Sheridan 

 Peak. 



Cyprina? iddingsi n. sp. 



PI. LXXIII, fig. 9. 



Shell small, convex, suboval in outline; beaks rather prominent, sub- 

 median; dorsal margin sloping gently from the beak to the posterior end, 

 slightly excavated in front of the beak and descending rather more rapidly; 

 anterior and posterior ends broadly and almost equally rounded; ventral 

 margin gently convex; posterior umbonal slope with a subangular ridge 

 extending from the beak to the postero-basal margin; surface with obscure 

 lines of growth and a few irregular concentric undulations near the free 

 margin. 



Length of largest specimen, 24.5 mm.; height, 18.5 mm.; convexity of 

 both valves, about 12 mm. 



This species differs from C. cinnabarensis in its smaller size, proportion- 

 ally greater convexity, more elongate form, narrower posterior end, less 

 prominent beaks, and more distinct umbonal ridge. Its generic position is 

 doubtful, as its hinge characters are entirely unknown. 



From saddle at head of Fawn Creek northeast of Monument Peak, 



