UPPER TRIASSIO SPECIES. 113 



however, resemble those of G. Blakei. The abdomen of the latter, however, 



is at first rounded, then more acute, or with a raised keel-line, and finally 



this disappears in a flattened abdomen." — (A. H.) 



Locality and position. — Buena Vista Canon, West Humboldt Range, 



Nevada; Trias. 



Gymnotoceras Blakei, Gabb (sp.)- 



Plate 10, figs. 10, 10<?, 10 6, 10 c; and pi. 11, figs. 6, Ga. 



Ammonites Blakei, Gabb (1864), Geol. Survey of California (Palaeout.), I, 24, pi. 4, figs. 

 14-15. 



Shell more or less compressed-discoidal, with periphery rather nar- 

 rowly rounded, or subangular, in consequence of a narrow, obscure, smooth, 

 welt-like ridge, usually seen along its middle, being sometimes more prom- 

 inent in the more compressed specimens ; umbilicus moderately deep, and 

 equaling generally a little more than one-fifth the breadth; volutions six 

 or more, more or less compressed laterally, particularly in adult specimens, 

 but usually more rounded in the young; each enveloping one-half to three- 

 fourths of the next within, rounding to the p^nphery, and truncated, or, in 

 adult specimens, even a little overhanging around the umbilicus; aperture, 

 as determined by sections of the volutions, subcordate, or, in compressed 

 specimens, subhastate, with the sinus on the inner side rather deeply defined. 

 Surface ornamented by generally distinctly-defined costse, which, in most 

 cases, increase by bifurcating (usually at a slightly pinched-up prominence 

 on the larger specimens) between the umbilicus and the middle, or in part 

 by the intercalation of others between; all curving strongly forward as they 

 approach the periphery, where they become obsolete before reaching the 

 faintly -marked central ridge. 



Septa with siphonal lobe rather short, or about as wide as long, and 

 divided at the extremity into two short, digitate, terminal branches, above 

 which there are two or three very small lateral serratures. First lateral 

 lobe, longer than the siphonal, and provided with two short, digitate 

 terminal divisions, with some very small lateral notches above. Second 

 lateral lobe considerably smaller than the first, but not differing very mate- 

 rially in other respects. Third lateral lobe still smaller than the second, 



and merely provided with two or three minute notches at the end ; between 

 8p R 



