460 University of California Publications in Geology [Vol. 8 



Posterior dorsal margin slightly flattened into an escutcheon-like area, 

 which is more prominent toward the base than in the vicinity of the 

 beaks. Surface sculptured by numerous, somewhat irregular, con- 

 centric lines. Hinge plate of left valve broad with three well devel- 

 oped cardinals ; posterior cardinal elongated but not prominent, with 

 small contiguous area of hinge plate roughened; anterior cardinal 

 rather thin and high ; anterior lateral strongly developed for this 

 genus. The writer was not able to get a good specimen of the hinge 

 of the right valve. Pallial sinus elongate, subacute. Adductor muscle 

 impressions strong, especially the anterior one. A fairly prominent 

 ridge is seen on the cast of the shell, extending down from the beak 

 about half way to the ventral edge, corresponding to a broad shallow 

 groove on the interior of the shell. 



The species was found in the same horizon with Dosinia merriami, 

 n. sp., var. diabloensis, n. var.. Upper San Pablo Group, but differs 

 from that form in that it is broader in proportion to the height ; it has 

 a longer anterior dorsal edge ; the lunule is not as broad ; the anterior 

 extremity is more produced ; the ventral edge is more regular and less 

 oblique anteriorly; also the posterior dorsal edge is slightly more 

 arcuate. The hinge plates of the two species are different ; the hinge 

 plate of D. arnoldi, n. sp., is narrower posteriorly and the posterior 

 dorsal cardinal of the left valve is less prominent and lacks the corru- 

 gations seen on the corresponding tooth of D. merriami, n. sp., var. 

 diabloensis, n. var. ; also the second cardinal of the left valve is less 

 elongated. Lastly, D. var. diabloensis lacks the marked groove seen 

 on the inside of the shell of D. arnoldi. 



Named in honor of Dr. Ralph Arnold, who has contributed so much 

 to the Tertiary palaeontology of California. 



Occurrence. — Prom the Upper San Pablo Group on the south side 

 of Mount Diablo, University of California locality 1604. Also found 

 in Santa Margarita beds to the north of Coalinga. 



DOSINIA (DOSINELLA) MERRIAMI, n. sp. 

 Plate 49, figures 1 and 2; plate 52, figures 1 and 2 

 Shell orbicular ; beaks rather prominent, strongly prosogyrous : 

 lunule deep and well defined. Proportion of height to length some- 

 what variable ; on some specimens the height and length are about 

 equal, while on others the height is greater than the length. Anterior 

 dorsal shoulder fairly prominent, sloping out nearly at right angles 



