MESOZOIO FOSSILS. 601 



The only marine Triassic fossils that have been found in the Rocky 

 Mountain region are from the Lower Trias, beneath the "Red Beds" in 

 southeastern Idaho. 1 The very few fossils that have been obtained from 

 the Red Beds farther south (in New Mexico and southern Colorado) seem 

 to be of fresh-water origin. 



In California and Nevada, however, marine Triassic beds are well devel- 

 oped, and have yielded a varied fauna which is as yet mostly undescribed. 



JURASSIC. 



The Jurassic fossils form much the largest and most important part of 

 the Mesozoic collection. The fauna is not large, but most of the species 

 are abundantly represented, and in number of species it compares favorably 

 with the Jurassic of other parts of the Rocky Mountain region. The col- 

 lections are from many localities in two general areas — one, which yielded 

 the most fossils, in the northwest corner of the Park, on the headwaters of 

 Gardiner and Gallatin rivers and near the Yellowstone ; the other on the 

 slopes of Sheridan Peak and farther southwest of Snake River. 



The fossils from all these localities evidently belong to a single fauna, 

 though two zones are recognizable, distinguished more by lithological 

 differences than by faunal peculiarities. The upper zone of arenaceous 

 limestone has yielded an abundance of Bhynchonella gnathophora, B. myrina, 

 Ostrea strigilecula, Camptonectes pertenuistriatus, C. beUistriatus, and a few other 

 forms. Most of these also occur in the underlying calcareous clays and 

 marly limestones associated with many other species, of which the most 

 abundant are Pleuromya subcompressa, Pholadomya kingi, and Gryphcea cal- 

 ceola var. nebrascensis. 



The same fauna is represented in the beds just beyond the northern 

 limits of the Park, at Cinnabar Mountain, where fossils that are included in 

 the present report were obtained by Dr. A. C. Peale in 1872. These were 

 identified and some of the species named by Prof. F. B. Meek, 2 but it was 

 not until 1880 that they were illustrated and more fully described by 

 Dr. C. A. White. 3 Still earlier Captain Raynolds had brought back Gry- 

 phcea calceola var. nebrascensis and a few other fossils of this horizon from 



1 See White, Triassic fossils of southeastern Idaho : Ann. Rept. U. S. Geol. Surv. Terr, for 1878, 

 pp. 105-118. 



2 Ann. Rept. U. S. Geol. Surv. Terr, for 1872, pp. 471-474. 

 ^Ideni for 1878, pp. 143-153, Pis. XXXVII and XXXVIII. 



