386 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Subgeneric characters: Form involute, laterally compressed, robust whorls, 

 deeply embracing, not deeply indented by the inner volutions. Sides con- 

 vex, abdominal shoulders rounded, venter high, and raised in the middle in 

 a keel-like ridge. Umbilicus moderately wide, exposing the inner whorls. 

 Surface ornamented with sigmoidal ribs, which branch on the sides, and 

 swing sharply forward across the rounded shoulders to the median ridge. 

 There are no umbilical nodes and none on the flanks, but in age the ends 

 of the ribs tend to form knots on the shoulders, although these are never 

 pronounced. The body-chamber seems to be long, nearly a revolution, but 

 the genus cannot be classed with the Tropitidae, on account of its ontogeny, 

 which is that of the Ceratitidae. 



Septa ceratitic, lobes serrated, and saddles slightly denticulated. 



Mo j sis o vies ^ says that Gymnotoceras is merely a synonym 

 of the group of Ceratites geminati, characteristic of the 

 Arctic Trias, but it seems to the writer that its characters 

 are sufficiently distinct from those of Ceratites nodosus 

 to warrant giving subgeneric rank to the group, which 

 would then include most species of the Ceratites geminati. 



As thus characterized, Gymnotoceras is diagnostic of the 

 Middle Trias, in which horizon it occurs in Nevada and in 

 northern Siberia. It does not seem to the writer, however, 

 that all species of the Ceratites geminati should be included 

 in Gymnotoceras, but only those with the sigmoidal lateral 

 ribs, raised keel, and absence of lateral spines. 



Ceratites (Gymnotoceras) blakei Gahb. 



Plate XLIII, Figs. 9-10; Plate XLIV, Figs. 2-3. 



1864. Ammonites blakei, Gabb, Pal. Calif., Vol. I, p. 24, pi. iv, figs. 14-15. 

 1877. Gymnotoceras blakei, Meek, U. S. Geol. Expl. Fortieth Parallel, 

 Vol. IV, p. 113, pi. x, figs. 10 a-c (not pi. xi, fig. 6). 



Form involute, robust, laterally compressed, whorl deeply embracing, but 

 not deeply indented by the inner volution. Sides flattened-convex, venter 

 high, and narrowed to a median keel. Abdominal shoulders narrow and 

 sloping in early maturity, more abrupt in later age. Umbilicus rather narrow 

 and deep; umbilical shoulders abruptly rounded, with steep inner walls. 

 The height of the whorl is about one-half of the total diameter, and the width 

 is three-fourths of the height. The indentation is one-fourth of the height. 

 The width of the umbilicus is about one-fifth of the total diameter. 



The surface is ornamented with strong radial dichotomous ribs that 

 branch on the flanks about one-third of the way from the umbilicus, and 

 bend sharply forward on the abdominal shoulders, becoming obsolete on the 



'See Bibliog. 20— p. 23. 



