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



The wing is of about the same size, but it is straight. The posterior 

 wing is broader and more distinctly marked than is usual in var. arcuata 

 in both valves, and especially long and prominent in the left valve. The 

 umbo is apparently less prominent in the left valve than in arcuata, and 

 that of the right valve is possibly less acute, but these characters are 

 rendered somewhat doubtful by the compressed condition of the fossils. 



It is obvious, as stated by Lahusen, that this is the representative form 

 of Aucella hronni. That species has the umbo of the left valve less promi- 

 nent than in most species of the same genus, and the shape of the hinge 

 area and umbo of the right valve is very similar to that of this species. 

 It is from the zone of Cardioceras alternans in the Oxfordian of Russia, 

 and is regarded by him as one of the oldest and most primitive of the 

 Aucellse. 



This fact is extremely interesting in connection with the supposed age 

 of the rocks of the locality given above, which also contains the Cardi- 

 oceras ditbium, a representative of Cardioceras alternans, and also because 

 of the following additional observations of Lahusen. This author quotes 

 his Aucella hronni as associated with Oppelia tenuilobata, Oppel, in the 

 Polish Jura, and also regards the A. mosquensis, Tullberg, from Nova 

 Zembla as identical with hronni, and says that it occurs with Cordioceras 

 alternans, and also that the Spitzbergen species is found in the same asso- 

 ciation as in Russia. This form also occurs at Texas ranch, Calaveras 

 count}^ California, and with it a variety still more like Posidonia, the 

 outlines less elongated, which I have labelled var. posidoniformis. 



Y elongata 



' aviculseformis 



Aucella erringtoni. Meek. Geol Cal., vol. I, appendix, pi. 1, fig 3 (not figs. 



1,2, or 4, 5). 

 Found only at locality 719, Texas ranch, Calaveras county (Meek's 



locality was Mariposa county). 



This variety is just intermediate between elongata d^ndi aviculseformis, and 

 differs from the latter only in having a narrow posterior wing. The out- 

 line given by Meek is an extreme form of this variety, in which the dorso- 

 ventral diameter is much shorter in proportion to the antero-posterior 

 diameter than in the normal forms. 



Var. Elongata orhicularis. 



This differs merely in being more circular in outline and approximates 

 toward Aucella orhicularis. 



