CRETACEOUS FOSSILS. 81 



Figures, natural size. 



Localities: Chico Creek; Pence's Ranch, Butte County; Cottonwood Creek, 

 Shasta County; Orestimba Canon, Stanislaus County ; in debris at Point Loma, 

 near San Diego (?) ; and near Martinez (Mathcwson). Only found in Div. A. 



The most common form is that found in the above localities, in the northern 

 part of the State, having few or no ribs, and with an outline more or less ap- 

 proaching an ellipse (Fig. 27). The costate form (Fig. 29), in which the dorsum 

 is more or less acute, is connected by imperceptible gradations with the other, and 

 differs from it in that, while every branch and spur of the septum agrees with the 

 diagram (Fig. 27 b), still the lobes are all somewhat more slender. The specimen 

 illustrated by Fig. 20 is from near Martinez, and is one of the best examples of 

 this form. 



B. (sp. indet.) 



PI. 17, Fig. 28, 28 a; PI. 14, Fig. 28 b. 



This form, labelled "Vancouver Island," in the collection of the California 

 Academy of Natural Sciences, is figured, in the hope that it may throw some light 

 on the determination of this truly difficult group of fossils. Externally, it cannot 

 be distinguished from the preceding, unless by its usually greater size, some speci- 

 mens being two inches in their dorso- ventral diameter. The lines of growth 

 appear to indicate somewhat shorter lips to the aperture. The differences in the 

 septum can be better understood by a comparison of the figures on pi. 14, than by 

 any description. In place of the tongue, in the middle of the dorsal lobe, there is 

 always a minute emargination. The relative sizes of the dorsal and lateral saddles 

 are well marked, as well as the proportionate size and general form of all the 

 lobes. The terminal branches of the superior lateral lobe are markedly different : 

 and the two large and two smaller teeth in the middle of the inferior lateral lobo 

 of the present form cannot fail at once to distinguish it. A careful comparison 

 of a large number of specimens of both the forms has failed to detect any im- 

 portant variation in any of these details. 



PAL. VOL. I. — 11 



