OLDER TRIASSIC DEPOSITS. 397 



Diller's geologic essay might be read in connection with the paleontol- 

 ogy, so far as his work and mine cover the same ground. 



The Trias. 



Discovery. — The abundance and good preservation of the fossils in the 

 Monotis bed of the Swearinger slates was made known by the survey of 

 California under J. D. Whitney, and they were accurately described by 

 Gabb in the first volume on the paleontology of California. The Hard- 

 grave sandstone was also found by this survey, and some of the fossils 

 of this bed were described by Meek in the same volume. One cannot 

 praise too highly the work of these explorers and authors when the great 

 difficulties under which they labored, both in the field and cabinet, are 

 taken into consideration. They established all that was desired at that 

 time, the demonstration of the presence of the Trias and Jura in the 

 Sierras ; and this primary fact and the publication of the fossils also led 

 to the explorations of which the results are given in this paper. If these 

 last are in their turn equally suggestive and useful to our successors, they 

 will have fulfilled all reasonable anticipations. 



Swearinger Slates: Monotis Bed. — The first and oldest fauna of the Trias 

 was found at the locality made known by the California survey near 

 Robinson's ranch/ 1 ' These slates were filled with shells of Monotis sub- 

 circularis, Gabb, a species so close to the typical M. salinaria, Schloth., 

 that 1 have grave doubts if it be really a distinct species. 80 tar, at least, 

 1 have failed in finding any differential characteristic. 



The fossils of M. subcircularis are closely compressed, and the species 

 grew in banks, as did its congener in limestones at Hallstatt, though 

 its habitat must have heen a clay bottom. The Monotis is accom- 

 panied by Pecten deformis, Gabb, which is, however, not abundant. 

 Hemientolium (Posidonomya) daytonensis, sp. Gabb,f is an equally rare 

 species, and Modiola triquaetrseformis is still rarer. 



!> lonella Bed. — In the upper part of the same slates and closely under- 

 lying the limestone there is a fauna differing somewhat from that of the 

 slates below, J comprising — 



Monotis subcircularis, Gabb, rare. 

 Daonella tenuistriata, n. sp.. rare. 



*This or some neighboring establishment was then called Gifford's ranch. 



fThis is the typi □ u genus, which I have called Hemientolium. The young until a com- 



itively late stage has the straighl hinge line gn en in Gabb's figure (Geol. Surv. Cal., Pal., vol. i, 



1864, pi. 6, fig. 32). Subsequentlj tin- anterior hinge I in 1 1 d iv> loped im 1 acute ascending wing 



mbling th< anterior win '.uliumcor p Quenst.) of the Jura, but no corresponding 



a sion of 1 he posterior h ing i« de^ eloped. 



rhi 1- provisionally called the Daom lla bed, bul it is no( ye( positively ascertained tli;>t the 

 fauna i- separable from thai "i the Rhabdoc* ra, I atone. 



