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



Aucella orbicularis. 



Aucella erringtoni, Meek. Geol. CaL, vol. II, appendix no. 1, fig. 4 (not 



figs. 1-3 or 5). 

 jo^^*^ Log., in Calaveras county, generally. 



This variety, as the name indicates, has a remarkably circular outline, 

 and the oral region is exceedingly gibbous and projects anteriorl}'- as in 

 Meek's figure ; but his figure is certainly an extreme form, resembling, 

 except in size, the young shells of the specimens from which this descrip- 

 tion is taken. All the good specimens examined by me are also right 

 valves, so that it is quite possible that Meek's extraordinary form may 

 be another variety, if not a distinct species. Right valves of this variety 

 can be easily mistaken for slightly distorted specimens of Amusium 

 aurarium in which the anterior wing has. been lost, so closely" do they 

 approximate in outline to Meek's figure of that form, and the shell also, 

 though usually marked by prominent lines of growth and striae, is some- 

 times quite smooth. 



The unequal umbo having the characteristic anterior reentrant curve 

 and wing of Aucella is, however, usually sufficiently well marked to pre- 

 vent an observer from making this mistake. This variety is certainly 

 very similar to the Aucella crassicoUis, var. solida in the shape of the 

 younger part of the right valve, but diff'ers materially in having stria- 

 tions and in the shape of the adult. 



One specimen, the right valve of a young shell of this variety (or at 

 least at present here included), has exactly the outline of Eichwald's 

 Aucella concenirica^ in the extreme variety, as given in his figure 2, show- 

 ing that a corresponding variety occurs in Alaska in this species. Never- 

 theless, Eichwald's figure ohows a smooth shell, whereas the valve of the 

 California species is strongly striated over the entire surface. Usually, 

 however, orbicularis approximates in the shape of the right valve to Eich- 

 wald's figures 5 and 6 of Aucella pallasi. The anterior curve of oral and 

 hinge region is not so protuberant and does not project dorsally, and the 

 valves are usually striated, left valve being entirely distinct. 



It is evident that this species, aviculseformis and elongata, belong to the 

 pallasi type, having broad hinge line and rotund oral areas in the right 

 valves, whereas arcuata belongs to the narrow hinged type of A. mos- 

 quensis, the umbo also being more nearly central in right valve than in 

 the former. 



* Mangischalk et Aleutian Inseln, pi. 17. 



