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



Aucella piochi, and are associated with AmmonitiDse of the group of 

 Perisphinctes of the Upper Jurassic facies, and so far not a single species 

 of this genus has been found in the Knoxville. The conclusion, there- 

 fore, that the Knoxville slates contain a distinct and younger fauna than 

 the Gold belt slates rests upon a very strong basis of paleontologic evi- 

 dence. 



Aucella elongata. — Aucella elongata found near Reynolds ferry is a broad 

 and also excessively elongated shell and very distinct from Aucella erring- 

 toni var. arcuata in the characteristics of the outline, the constancy of the 

 longitudinal striation, and the shape of the hinge line in the right valve. 

 The representative form of this species, as stated by Lahusen, in Russia 

 may be Aucella bronni from the zone of Cardioceras alternans, in the Upper 

 Jura of Russia. 



Aucella aviculasfonnis, from several places in Calaveras county, has an 

 outline somewhat like Avicula, with a broad posterior wing. The young 

 shells are similar to those of Aucella obicularis, but they become more 

 * elongated by growth. Some of Eichwald's species from Alaska,^ as fig- 

 ured by him, certainly appear to have been identical with this California 

 shell in outline, but they are not striated. There may be close affinity 

 between this species and Aucella mosquensis, Whiteaves, of Queen Char- 

 lottes island, as stated by Lahusen, and also with Volgensis, Lahusen, 

 but in all of these supposed congeners longitudinal striae are absent in 

 the drawings, at least, and the posterior wings of the valves are not well 

 developed. 



Aucella obicularis. — Aucella obicularis^ from all the mentioned localities 

 in Calaveras county, has an extremely circular outline, and the hinge 

 area of the left valve is correspondingly modified, the anterior part being 

 very gibbous and protuberant. Eichwald f has figured a similar fopii 

 from Alaska, but here again our species difi'ers in being striated. 



Conclusions based on Study of Aucella. — The evidence gathered from the 

 study of Aucella shows that, while forms of this genus in the California 

 slates of the Mariposa series are all more or less distinct on account of the 

 prevalence of the strong longitudinal striae, they represent two groups of 

 Aucellae as they appear in Europe. It is obvious that the forms which I 

 have named Aucella orbicularis^ aviculseformis and elongata have a broad 

 hinge and gibbous oral or anterior outline in the right valve like that of 

 the pallasi type, whereas Aucella erringtoni var. arcuata has a narrow 

 hinge line, with the umbo more nearly central, as in the Mosquensis type 

 of that genus, and it is the latter alone which approximates in outline 

 to the narrow^ form oi Aucella piochi, Gabb, prevalent in the Knoxville 

 series. 



*Aucelln. palnssi, Eichwald: Mangischalk et Aleuts Inseln, pi. 17, i". 3-4, and Mosquensis, f. 13-14. 

 fMangischalk et Aleuts luselu, pi. 17, f. 2. 



