74 PALAEONTOLOGY OP THE UPPER MISSOURI. 



This species bears some general resemblance to 0. Marshii, of Sowerby, but is a 

 thinner and more compressed shell, with less prominent, and much less angular 

 plications or costse. Its area is also proportionally much shorter. 



Locality and position. Jurassic beds at Red Buttes, on the north branch of 

 Platte River, Dakota Territory, lat. 42 50', long. 106 40' west. Collected by the 

 Expedition under the command of Capt. J. H. Simpson, of U. S. Top. Engineers. 

 (No. 1884.) 



Genus GRYPH^EA, LAMARCK. 



Synon. Auricularia, Auriculites, &e. (sp.), of LLHWYD and other early writers. 



Grypheea, LAMABCK, Syst. An. 1801, 398. EOISSY, Mol. 1835, 202. BLAINV. Die. Sci. Nat. t. 19, 1821, 

 p. 533. Risso, Hist. IV, 1826, 290. 



Griph&a, BLAINV. Malaool. 1825, 522. 



Pycnodonta, FISCHER DE WALDIIEIM, Bull. Mosc. VIII, 1835. 

 Etym.ytlj., a Griffin. 

 Examp. Gryphsca arcuata, LAMK. 



Shell generally free, especially in the adult state. Lower valve deep; beak 

 prominent and distinctly incurved, and but slightly oblique. Upper valve flat 

 or concave ; beak usually truncated. Hinge, ligament, and muscular and pallial 

 impressions as in Ostrea. (Animal unknown.) 



The shells of this genus differ from those of the true Oysters in being more 

 regular, in having the lower valve deeper, and particularly with its beak more 

 prominent and incurved. They seem also to be scarcely ever plicated as we often 

 see in the genus Ostrea. From Exogyra they differ mainly in having the beak of the 

 lower valve curved upwards and inwards, instead of to one side, as well as being 

 prqbably always without the large plications sometimes observed in Exogyra. Most 

 of these distinctions, however, particularly the prominence and incurving of the 

 beak, sometimes become so faintly marked that it is not always easy to separate 

 the species of these three groups. 



This type appears to have been first introduced during the deposition of upper 

 members of the Triassic series ; at any rate, a few species have been referred to it 

 from the St. Cassian beds of the Tyrol usually referred to that epoch. It is more 

 frequently met with in the Liassic and other members of the Jurassic system, and 

 probably attained its maximum development during the deposition of the Cretaceous 

 rocks. A few species have been referred to this genus from the Tertiary rocks of 

 Europe ; though it is doubtful whether or not they are true Gryphseas. No living 

 examples of the group are known. 



Gryphtea calceola, var. nebrascensis. 



(PLATE III, Fig. la, b, c, d, e, and annexed cuts.) 



Gryphsea calceola, QCENSTEDT, Handb. Petref. tab. 40, fig. 29-31. QUENSTEDT, Der Jura, 1856, 352, tab. 48, figs. 



2, 3, 4, and 5. MEEK, MSS. Capt. Simpson's Kept. Utah.' 

 Gryphsea calceula, var. iiebrascensis, MEEK & HAYDBN, Proceed. Acad. Nat. Sci. Phila. Dec. 1861, 437. 



Shell, when normally developed, snbovate in small specimens, but becoming more elongate and proportionally 

 narrower with age. Under valve very thick in the umbonal region ; beak prominent, narrow, produced, and 

 strongly incurved ; anterior side with a more or less denned sulcus, which never extends quite to the point of the 



