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University of California Publications in Geology [Vol. 9 



other muscle scars deeply impressed. The hinge of the type lacks 

 the posterior laminae and the posterior arm of the cardinal. Hinge 

 plate rather heavy, chondophore relatively shallow, roofed ; right 

 cardinal with a small anterior arm which is closely appressed to the 

 dorsal margin of the shell ; anterior laminae long, strongly striated 

 on their inner sides, the dorsal one being nearly as long as the 

 ventral one, and arising as a long thin tooth, from the hinge plate 

 to a height greater than that of the adjacent shell margin. In 

 one specimen the ventral lamina is rather heavy and distinctly 

 striated, not confluent with the anterior arm of the cardinal. The 

 cotype shows the left valve with a rather small but prominent car- 

 dinal and large striated laterals, which are distant from the urn- 

 bones, the anterior one not reaching to the cardinal arm. The 

 dimensions of the type are : height, 43 mm., convexity, 18 mm. 

 Those of the cotype are : length, 54 mm., height, 48 mm., convexity, 

 36 mm. 



This species is extremely variable in general shape, paralleling 

 in many respects the forms assumed by Mulinia densata. It differs 

 from all inulinoid species of the Coast region in the striated char- 

 acter of its laterals and laminae. 



The type was obtained at University of California Locality 1946, 

 which was described by J. P. Buwalda in the following words: 

 "About 150 feet slope distance up the south-west side of the high- 

 est hill north-east of Joe Mendosa's house, which is about one-half 

 mile south-west of Shell Ridge, Concord Sheet, Contra Costa County, 

 California." 



Occurrence. — Miocene: Upper Monterey to Upper San Pablo. 

 Pound at a number of localities in the region of Mount Diablo, 

 California. 



Generic Position Uncertain 



MACTRA(?) GIBBSANA Meek 



Mactra gibbsana Meek, Proc. Phila. Acad. Nat. Sci., vol. 13, p. 315, 

 1861; Bull. U. S. Geol. and Geog. Surv. Terr., vol. 2, no. 4, p. 374, pi. 2, 

 figs. 8, 8a and 86, 1876. 



Meek described this species as follows: 



Shell transversely oval, or subtrigonal, moderately convex, rather thin; 

 anterior side narrowly rounded; base forming a regular semielliptical curve; 

 posterior side slightly truncated at the immediate extremity, abruptly 



