Clark: The San Lorenzo Series of Middle California 157 



diagnostic characters. It is questionable whether S. sicareus goes 

 below Upper Miocene (San Pablo, Santa Margarita). The species 

 which is most common in the Upper Miocene and Lower Pliocene 

 (San Pablo, Etchegoin) on the West Coast is S. perrini B. L. 

 Clark; 126 this appears to be more closely related to S. rosaceus Cpr. 127 

 than to S. sicareus, under which name it has been listed a great many 

 times. S. perrini differs from S. rosaceus in being heavier and the 

 proportion of height to length is greater. The hinge plate of S. 

 perrini has not been described. 



S. gravidus differs from S. rosaceus in that it is heavier; it has 

 a different proportion of height to length, and the anterior end is less 

 oblique ; the cardinals of the two species are quite different. 



S. gravidus differs from S. perrini in the following respects : the 

 anterior end is not so oblique, being more regularly rounded ; there is 

 usually more of a margin in front of the sinus on the anterior end. 



S. gravidus is very similar in outline to S. parallelus Gabb. 12S It 

 differs from this species in that the anterior end is not so obliquely 

 truncated and the anterior margin not so strongly depressed. 



There is a Solen in the Lower Miocene beds of Kern River section 

 near Bakersfield which is very similar to, if not identical with, S. 

 gravidus; the writer would like, however, to see more and better 

 material before deciding definitely as to its identity with S. gravidus. 



Family Mactridae 

 Genus SPISULA Gray 

 SPISULA OCCIDENTALIS Gabb 

 Plate 11, figure 1 



Hemimactra occidentalis Gabb, Geol. Surv. Calif., Palaeontology, vol. 2, 



p. 54, pi. 15, figs. 13, 13a, 1869. 

 Spisula occidentalis Gabb, Clark, Univ. Calif. Publ., Bull. Dept. Geol., 



vol. 9, no. 2, p. 15, 1915. 

 Spisula occidentalis Gabb, Packard, Univ. Calif. Publ., Bull. Dept. Geol., 



vol. 9, no. 15, pp. 288-289, pi. 23, figs. 1, 2 and 3, 1916. 



This is a very common species in the Agasoma gravidum zone. It 

 is somewhat similar in outline to S. catenulatus Conrad, a Recent 

 West Coast species. 



126 Univ. Calif. Publ., Bull. Dept. Geol., vol. 8, no. 22, p. 477, pi. 44, fig. 2, 1915. 



127 For good description of Solen rosaceus see Arnold, Mem. Calif. Acad. Sci., 

 vol. 3, p. 171, 1903. 



128 ;F or original description of Solen parallelus, see Geol. Surv. Calif., Palaeon- 

 tology, vol.. 1, pp. 146-147, pi. 22, fig. 117, 1866. 



