134 PALAEONTOLOGY OF CALIFORNIA. 



the external face of the shell, and leave corresponding grooves 

 on the cast. 



Septum composed of a dorsal, superior and inferior lateral, and 

 four supplementary lobes. Dorsal lobe long, bifurcate for two- 

 fifths of its length, with two small points over the siphuncle ; 

 the branches bearing two spurs on the outer side; above the 

 branches are one large and two smaller spurs. Dorsal saddle 

 deeply divided into three irregular branches. Superior lateral 

 lobe pretty symmetrically trifurcate; the upper lateral branch 

 being a little more complex than the lower; two spurs above on 

 each side. Lateral saddle like the dorsal, but a little smaller. 

 Inferior lateral lobe much smaller than the superior, slender, 

 unsymmetrical, obliquely bifurcate, near its end, and bearing a 

 large branch on the upper side near the middle. Supplementary 

 saddles bifurcate; supplementary lobes rapidly diminishing in 

 size to the umbilical angle, and resembling in fundamental struc- 

 ture, the inferior lateral. 



Diameters : greater, 1.6 inch ; lesser, 1.25 inch ; diameter of umhilicus, .2 inch ; 

 height of aperture from umbilical angle, .9 inch ; width, .65 inch. A specimen 

 from Benicia gives for the greater diameter, 2. inch ; lesser, 1.5 inch ; umbilicus, 

 .28 inch ; height of aperture as above, 1. inch; width, 1. inch. 



This shell, though one of the most inconspicuous, is nevertheless one of the most 

 interesting of the California Ammonites, being the only member of the family- 

 known to occur at the same time in the Martinez and Tejon Groups. It is found 

 at Martinez and Benicia in the former group, and I have collected it in the latter, 

 in place, at the very top of the series at the coal-mines near Clayton. Plate 10, 

 figs. 5, and 6 b, Cal. Report, Pal., Vol. 1 , are from a specimen from above the coal- 

 beds on Coal Mine Hill, near Mount Diablo. 



A. WlIITNEYI, n. S. 



PI. 22, Fig. 14, a, b. 



Shell discoidal, whorls about six, rounded, slightly embracing, 

 increasing gradually in size; sides rounded, slightly converging 

 towards the dorsum, which is regularly arched ; umbilical margin 

 abruptly rounded; umbilicus broad, shallow. Aperture sub- 



