FOSSILS OF THE JUIJASSIC TERIOD. 203 



strongest near the middle of the basal border, and decrease very slightly in 

 size anterior to this point; but posteriorly they decrease more rapidly, and 

 on and near the posterior alation arc quite fine. Interspaces narrow and 

 concave. 



This species is closely allied to Lima I'meata Gold, (rz Plagiostoma Jincatd 

 of authors,) but differs in being more regularly rounded on the basal and })os- 

 tero-basal borders, as well as in being more erect, that species having con- 

 siderable obliquity, the longest point of the base being much nearer the 

 anterior side. The beak is also shorter, and not nearly so prominent, and 

 th(} greatest convexity of the shell nearer the anterior margin, while that 

 one is most prominent near the middle of the valve, and nearly evenly 

 roinided in both directions from this point. The posterior wing of this 

 species is more alate than in that one, and more finely ribbed, giving about 

 fifteen additional ribs oyer and above the number counted on that s])ecies. 

 This character is perhaps the most distinctly marked feature of the species, 

 and one that will serve to readily distingui^;h them, although the general 

 resemblance of the shells is \e.Yy great. 



Formation and localUij. — In cherty limestones of Jurassic age, at Flam- 

 ing Gorge, Uinta Range, Utah. Collected by S. F. Emmons, esq. 



Genus TRIG ONI A Brug. 



TUICfONIA QUADKANGULARIS U. sp. 

 riate VII, fig. 22. 



Shell small, subquadrangular in outline, with depressed convex valves 

 and flattened inconspicuous beaks; length of the shell a little greater than 

 'the height; anterior end broad and rather squarely truncate; the niai'gin 

 but slightly rounded; basal line somewhat straightened or but slightly con- 

 vex, more strongly I'ounded behind; posterior end obliquely truncate, long- 

 est below; cardinal border short, not more than two-thirds the length of 

 the valve, very little concave, and subparallel with the basal nuirgin. 



Surface of the valve marked by a flattened, distinctly elevated ridge, 

 which rises from behind the beaks, and passes along the nmbonal slope, 

 extending beyond the posterior margin of the valve in a squarish projec- 

 tion, equal in extent to the elevation of the ridge. . Above the ridge, the 



