1922] Trash: The Briones Formation of Middle California 147 



The geographic distribution of the Briones formation appears to be 

 somewhat different from that of the Lower San Pablo, for on the north 

 side of Mt. Diablo the Lower San Pablo is present, while the Briones 

 is absent. 



Hence from (1) the faunal distinctness of the two formations, 

 (2) the different geographic distribution, and (3) the lithologic 

 difference with erosion contacts, it is concluded that the Briones is a 

 minor cycle of deposition distinct from the Lower San Pablo : 



The San Pablo group would accordingly consist of three minor 

 epochs, Briones, Lower San Pablo (Cierbo), and Upper San Pablo 

 (Santa Margarita) . 



DESCRIPTION OP SPECIES 

 Subkingdom MOLLUSCA 

 Class PELECYPODA. 

 Family Ledidae 

 Genus LEDA Schumacher 

 LEDA FURLONGI, n. sp. 

 Plate 1, figures la and lb 



Type.— No. 12362; cotype— No. 12363, Univ. Calif. Mus. Pal. 



Shell small, subovate, moderately ventricose; beaks obscure, nearly central; 

 slightly opisthogyrous; posterior dorsal edge gently concave; anterior dorsal 

 edge nearly straight; anterior end regularly rounded; posterior end subacutely 

 rostrate; lunule and escutcheon elongate, lanceolate, extending almost the entire 

 length of the dorsal margins, and rather strongly pouting; surface sculptured 

 with numerous fine regular concentric lines; hinge plate unknown. 



Dimensions. — Type specimen U. C. no. 12362; length, 20.8 mm.; alt., 11.3 mm.; 

 thickness of both valves, 8 mm. Cotype, U. C. no. 12363, length 16.1 mm.; alt., 

 8.6 mm. 



Occurrence. — Briones formation, U. C. loc. 15. 



Named in honor of Mr. E. L. Furlong of the Museum of Paleontology, Uni- 

 versity of California. 



L. furlongi, n. sp., somewhat resembles L. taphria Dall, 16 but 

 differs from the latter in being narrower, more elongate posteriorly, 

 possessing finer concentric sculpturing, and the lunule and escutcheon 

 being more strongly pouting. It differs from L. ochsneri Anderson 

 and Martin 17 in being less acutely elongated posteriorly, the posterior 

 dorsal slope being less concave, and the concentric ribs being finer. 



is Dall, Nat. Hist. Soc. Brit. Columbia, Bull. no. 2, p. 7, pi. II, figs. 6-8, 1897. 

 17 Anderson and Martin, Cal. Acad. Sci., ser. 4, vol. 4, p. 53, figs. 8a, 8b, 8c, 

 1914. 



