410 A. HYATT — TRIAS AND JURA IN THE WESTERN STATES. 



a species of the same group as Perisphinctes of the Solenhofen slates, and 

 CEcotraustes denticulata, another prevalent form in the youngest faunas of 

 the Jura in Europe. This association is again similar to that of the 

 faunas in Russia, since the Volgian, being in the Portlandian and occur- 

 ring just above the Oxfordian in that country, is equivalent, so far as 

 position is concerned, in part at least to the Solenhofen slates. This 

 comparison is sustained by the obvious fact that in the fauna of this and 

 other similar localities the genus Perisphinctes is the prevalent form of 

 the Ammonitinae associated with Aucella, as is also the case in the Rus- 

 sian fauna supposed to be of corresponding age. 



I have thought it best, as may be seen above and further on, to con- 

 sider the rocks of the western slopes of the Sierra Nevada containing 

 Perisphinctes, such as those near Greenwood and Colfax, as provisionally 

 of the same age, although they contain no Aucell^e. The relations of the 

 faunas of localities mentioned presents also, as regards the intimate rela- 

 tions and probable chronology of the appearance of the different species 

 of Aucellae, a remarkably close parallel with what Lahusen has worked 

 out in Russia. 



The Stanislaus river locality has only Aucella elongata, aviculseformis 

 and orbicularis, all of which belong to the Pallasi type of Aucella, that 

 which appears first in Russia. At the locality on the south bank of 

 Tuolumne river Aucella erringtoni var. arcuata makes its appearance, and 

 this is the representative of the Mosquensis type, which also in Russia 

 appears after pallasi and bronni. 



The foregoing are merely tentative comparisons, so far as the relative 

 chronology of the beds of the Gold belt slates are concerned. In order 

 to give a final opinion more materials are necessary and some of the 

 geologic details of the relations of the rocks in these difi'erent localities 

 should also be ascertained. They will, however, it is hoped, serve to 

 draw the attention of geologists and collectors to the need of making col- 

 lections as extensive as possible in every localit}^, and to the importance 

 of noting the exact horizon of each fossil, even though it occurs in imme- 

 diately contiguous and conformable strata in the same bank or quarry 

 and appears to them in the field to be the same species. 



Classification of the Jura. 



The full classification of the parts of the Jura in this country remains, 

 of course, to be worked out, but for general purposes and as a provisional 

 guide I have found that the general classification given by Steinmann^ is 

 the most satisfactory. Probably no strictly European classification will 

 suffice, but it seems clear that we can distinguish in the system of the 



*Elemente der Paleontologie, Leipzig, 1890. 



