438 PROCEEDINGS OF THE ACADEMY OF 



them the beak anything like near so strongly incurve! as Quenstedt's fig. 1, 

 tab. 48, of his Der Jura. 



Amongst the specimens now under consideration, belonging to Capt. Rey- 

 nolds' collections, from the Jurassic beds at Big Horn, Mountain Lat. 43 30' N., 

 Long. 108 West, we have a large series agreeing in nearly all respects with 

 the forms brought in by Capt. Simpson. Most of them would be called true 

 oysters, were it not for the fact that we find them occasionally shading into 

 Gryphce '-like forms. 



From another locality at the base of the mountains near the head of Wind 

 lliver Valley, a large number of specimens were obtained by Capt. Raynolds' 

 expedition, from the same horizon as those mentioned above, (and associated 

 with the same group of fossils,) nearly all of which present the form, and all 

 the characters of true Gryplueas, and appear to agree very closely with Quen- 

 stedt's fig. 1, on the plate above cited. Whether or not all these Nebraska 

 forms should be included in one species, as Quenstedt has done with the Ger- 

 man specimens, is an exceedingly difficult question to decide ; nor can we de- 

 termine very satisfactorily without authentic European specimens for com- 

 parison, whether or not our group of forms are really in all respects identical 

 with those figured by Quenstedt. Until these questions can be determined 

 from more satisfactory data than we now have at our command, we would 

 propose to designate the narrow shells with a distinctly incurved umbo from 

 Wind River Valley, as Gryphrea calceola, var. Nebrascensis, since they may 

 possibly be distinct from Quenstedt's species. 



These latter may be described as follows : lower valve very narrow, elon- 

 gate, arcuate, in old individuals thickened near the umbo, and provided with 

 an obscure sulcus from near the beak along near the anterior side to the ven- 

 tral margin ; beak slender, distinctly incurved, and directed a little obliquely 

 towards the front, often rather pointed, but sometimes slightly truncated at 

 the apex ; area triangular, arcuate, extending close up under the curve of the 

 beak, and provided with a shallow mesial depression ; muscular scar small, 

 shallow, oval, and located near the left or anterior side, surface ornamented 

 on the gibbous back of the umbo by distinct, irregular, radiating stria?, usually 

 extending to near the middle, on mature specimens, while the space between 

 this and the ventral margin is marked only by moderately distinct concentric 

 stria?, and stronger ridges of growth. 



Upper or smaller valve ovate, nearly flat on the outside, or a little convex 

 at the beak, and more or less concave near the middle, usually concave within 

 towards the cardinal extremity, which is truncated and thickened ; surface 

 ornamented with rather distinct marks of growth. 



Length from the most prominent part of the umbo, to the ventral extremity, 

 270 inches ; breadth at the extremity opposite the beak, about 1*20 inch ; 

 convexity, 0'73 inch. 



The most marked features of this shell of which we have before us more 

 than one hundred specimens, are its slender form, and the distinctly striated 

 character of the umbonal region of its lower valve. Its .greatest breadth is at 

 the ventral extremity, from which it narrows gradually towards the beak, the 

 anterior side being a little more expanded than the other, and in the lower 

 valve somewhat lobed in front of the sulcus extending from near the beak to 

 the ventral margin. The radiating stria? on the umbonal region of the under 

 valve seem to have commenced almost with the growth of the shell, and con- 

 tinued until it had obtained nearly half its full size, after which only con- 

 centric markings were developed. 



No one could for a moment confound any of the forms we have been describ- 

 ing with G. Pitcheri, (= G. dilatata of Marcon, not of Sowerby), even after 

 merely glancing at the specimens. None of those having the beak pointed 

 and incurved are ever one-half as broad, or deep in proportion to their length 

 as that shell, while they differ entirely in their surface stria?. In short, they 

 all differ as widely from that shell as any two species of the genus can perhaps 



[Dec. 



