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 lohes. 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, oblicptcly 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, l.G inch ; lesser, 1.25 inch ; diameter of umbilicus, .2 inch ; 

 height of aperture from umbilical angle, .9 inch; width, .Go 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. Beport, Pal., Vol. 1, are from a specimen from above the coal- 

 beds on Coal Mine Hill, near Mount Diablo. 



A. Whitneyi, n. s. 



PL 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- 



