126 KANSAS UNIVERSITY QUARTERLY. 



terior wing is small, and rounded toward the beak. The surface is 

 bent toward the opposite valve so as to leave a small opening between 

 the valves near the anterior margin. The posterior wing is larger, 

 and the surface is flat near the anterior margin. The surface of the 

 shell is ornamented with fine radiating plications, which are crossed 

 by concentric strite, producing small nodes at the points of intersec- 

 tion. These nodes are prominent near the basal margin in most 

 specimens, but rarely occur in the region of greatest convexity, which 

 is nearly always smooth. The surface markings are not so conspicu- 

 ous on the right valve. The anterior wing of this valve is separated 

 from the body of the shell by a very narrow slit or groove. 



Geological horizon : This species occurs in the sandstone of No. 

 10, and also in the nodules of No. 13. In the first stratum it has no 

 associates. In the latter, it has associated with it the fossils already 

 enumerated from that horizon. 



Locality : The Freeze-out Hills, and Como Bluffs, Wyoming ; also 

 occurs in Black Hills. 



Remarks : This fossil marks the lowest limit of the fossiliferous 

 belt of the Jura-Trias rocks of this region. It occurs in a thin stratum 

 of limestone intercalated between beds of shale. Below the limestone 

 stratum appear the characteristic Red Beds of the Permo-Trias. 

 Above are the fossiliferous beds of the Jura. 



Phol((d(nnffa rohusfo, n. sp. Plate XXVI, figs. 5, 6. 



Shell convex, elongate-oblong in general outline. The posterior 

 umbonal region is the area of greatest convexity. The cardinal line 

 is long and moderately straight. The beaks project a little beyond 

 the cardinal line and are slightly incurved. The dorsal border is 

 broadly convex. The posterior and the anterior margins are more 

 abruptly rounded. 



The surface of the shell is ornamented by several costae, radiating 

 from the beaks and curving posteriorly as they pass to the dorsal bor- 

 der. The costpe are crossed by prominent concentric undulations. 



Dimensions: Length, 26 mm.; width, 20 mm.; height, 8 mm. 



(xeological horizon : The species occurs in the abundantly fossilif- 

 erous stratum, No. 13 of the section. It is associated with Pinna 

 king I, Belemnites densus, Ta?icredia magna, Astarte packardi, 

 Pleurortiya compressa, etc. 



Locality : The Freeze-out Hills, Wyoming, 



Remarks : The above-described species differs from Pholodomya 

 lingi, described by White, in the matter of size and surface orna- 

 mentation. It does not appear to have any very close European 

 affiliations. 



