194 PALAEONTOLOGY OF CALIFORNIA. 



Length, 3.3 inches ; width, 2.8 inches ; height of single valve, .8 inch 

 A single cast in a soft, gray, fine-grained sandstone, from near Folsom, and 

 probably of the age of the Chico Group. Found by Professor Whitney. I have 

 seen in this same matrix, Ammonites Chicoensis, A. Suciaensis, Baculites Chicoensis, 

 and casts of some undetermined Gasteropods. 



AUCELLA, Keyserling. 

 A. PIOCHII, Gabb. 



PI. 32, Fig. 92, a, b, c. 

 (Inoceramus Piochii, Gabb ; Pal. Cal., Vol. 1, p. 187, pi. 25, fig. 173 (exclus. fig. 174).) 



This shell was originally described from two or three small specimens from near 

 Mount Diablo ; the Tuscan Springs specimens being the Mytilus, figured at 174, 

 and which I have separated in the present paper. During the last three or four 

 years, the species has been discovered in profusion in a large number of locali 

 ties, and proves to be of great value in the identification of one of the members 

 of the Shasta Group. Its specific characters may be defined as follows : 



SHELL variable in outline, convex, variably inequivalve ; sym 

 metrical and obliquely ovate in the young state, when both valves 

 have a small, narrow ear on the posterior side, which becomes 

 obliterated as the shell grows larger. In the adult, large valve 

 long, narrow, tapering, irregular in form; beak large, and 

 strongly incurved, often overhanging the opposite valve. Smaller 

 valve more or less triangular, anterior side usually somewhat 

 truncated; beak small, acuminate and pointed in advance; pos 

 terior cardinal margin pretty regularly arched and sloping down 

 wards rapidly; under the beak, on the anterior face, is a deep 

 indented fold of the shell, forming an internal sinus (seen in figs. 

 92, and 92 c). Surface of both valves very variable in convexity, 

 and in the adult, covered by irregular wrinkles, besides fine, and 

 in some cases pretty regular, concentric ribs. 



Figures, natural size, except 92 c, which is twice the size of the original. 



Very characteristic of a series of shales of the Shasta Group, found from Mount 

 Diablo, at various points along the east face of the Coast Range to the north end 

 of the Sacramento Valley. Two or three good specimens from Washington Ter- 



