130 PALAEONTOLOGY OF CALIFORNIA. 



AMMONITES, Brug. 

 A. Breweb.ii, Gabb. 



PI. 20, Fig. 5; PL 19, Fig. 5 a, I>, 6, 6 a. 



(Ammonites Brewerii, Gabb ; Pal. Cal., Vol. 1, p. 62, pi. 10, fig. 7.) 

 (A. Newberry anus, Gabb; (pars) loc. cit., PI. 10, fig. G.) 



This shell was originally described from several imperfect 

 casts in sandstone, showing nothing but the surface characters, 

 and not all of those in a perfectly satisfactory manner. Subse- 

 quent research has placed me in possession of abundant material 

 for redescribing the species. 



Shell discoidal, compressed; whorls increasing in size with 

 moderate rapidity, each whorl embracing about one-half of the 

 preceding one; sides flattened, converging convexly towards the 

 dorsum, which is narrow^ rounded; umbilical margin of the 

 whorl abruptly truncated at nearly a right angle. Mouth semi- 

 elliptical, the dorso-ventral diameter twice the transverse, the 

 ventral emargi nation, produced by the encroachment of the pre- 

 ceding volution, being about one-fifth of the greater diameter. 

 Umbilicus nearly one-fourth the diameter of the shell. Surface 

 variable from nearly smooth, or marked only by sinuous striae, to 

 strongly costate, the striae assuming the character of irregular 

 ribs, most marked near, and on the dorsum. 



Septum : Dorsal lobe small, bifurcate a little below the middle, 

 and bearing two points in the notch; each branch carrying two 

 spurs on the outer side; above the bifurcation are a large and a 

 small spur on the main lobe. Dorsal saddle broad, divided into 

 two unequal parts, the lower of which is much more deeply sub- 

 divided than the upper. Superior lateral lobe longer than the 

 dorsal, trifurcate, the lateral branch on the upper side arising 

 somewhat higher than the corresponding one on the other side, 

 and being much more subdivided ; above the origin of these 



