GEOLOGICAL SURVEY OF THE TEERITOEIES. 375 



outline, instead of sinuous ; anterior ear longer and more angu- 

 lar, compressed, but more distinct from the slight swell of t^ie 

 umbo than the other, and having its margin below broadly and 

 rather slightly sinuous ; surface ornamented by compressed, 

 generally simple, alternately smaller and larger radiating costee, 

 only the latter of which reach the beak, while those of both 

 series become nearly or quite obsolete on the ears, particularly 

 on the posterior one ; lines of growth small, rather regular and 

 obscure. (Eight valve unknown.) 



This is probably neither a true Pseudonionotis, nor an Aviculo- 

 jaecten ; but as I know nothing of the nature of its hinge, nor 

 of its right valve, its true generic characters remain doubtful. 

 Many paleontologists refer such forms to the genus Pecten, but 

 they are evidently distinct from that groui> as typified by the 

 existing P. maximus. Lincoln Valley, near Fort Hall, Idaho^ 

 9. Pinna (a smooth attenuated species) j shore of lake, twelve miles 

 from Fort Ellis. 



10. Modiola ; same as last. 



11. Myacites (Pleuromya) ; same as last. 



12. PJwladomya ; same as last. 



There are in the collection imperfect casts of several other bivalves 

 from the Jurassic beds; but they are not in a condition to be referred 

 to the proi)er genera. 



CRETACEOUS StECEES. 



1. Ostrea glcibra, M. & H.? ; Point of Eocks^ Union Pacific Eailroad, 



Wyoming ; from above a coal-bed. 



2. Ostrea Idriaensis, Gabb ?? ; Point of Rocks, Union Pacific Eailroad, 



Wyoming; from above a coal-bed. 



This i's a rather large compress, moderately thick, suinwate, or 

 ovate-subtrigonal species, with more or less pointed, undistorted 

 beaks, a comparatively small ligament area, a nearly flat upper 

 valve, and a shallow lower one. In the latter the ligament area 

 has a rather deep, mesial furrow, while the corresj)oudiug ridge 

 in the area of the other valve is usually quite prominent at its 

 inner end. The surface of both valves is only marked by dis- 

 tinct, more or less imbricating laminae of growth. 



I have had specimens of this species under consideration, 

 among the collections brought in by different exploring parties, 

 for some years past, but I have never been able to arrive at a 

 satisfactory conclusion in regard to it. It seems to be related 

 to 0. Idriaensis and 0. Breweri of Gabb, but 1 am by no means 

 sure that it belongs to either of them. If distinct, it may be 

 called 0. Wyomingensis. 



3. Ostrea (fragments) ; mouth of Warm Spring Creek. 



4. Anomia ? grypJiorhyuchus, Meek ; Point of Eocks, Wyoming. 



Shell small, rather thin, not very distinctly pearleaceous,, 

 ovate to suborbicular in outline. Upper valve, verj^ convex and 

 evenly rounded; beak marginal, prominent (the whole valve 

 tapering toward it), and curved upward ; surface, smooth, or 

 sometimes showing faint traces of radiating strise; muscular 

 impression, very obscure; lower valve unknown. 



This little shell rarely attains more than three-quarters of an 

 inch in length, with a breadth of about 0.60 inch, and a depth 

 of 0.33 inch. Some examples are proportionally wider and less 



