292 University of California Publications in Geology IVol. 9 



mnbones ; anterior dorsal edge straight ; posterior dorsal edge con- 

 vex ; sloping regularly to the broadly rounded extremity ; base 

 arcuate. Hinge plate wide, chondophore deep, not roofed ; left car- 

 dinal prominent, extending from the dorsal to the ventral margin 

 of the plate ; arms of equal length, heavy, the posterior one bearing 

 an indistinct lamella ; ventral sinus narrow ; laterals long, the ante- 

 rior one arising from the ventral edge of the plate ; it is nearly 

 flat-topped, and is not in alignment with, nor reaching to the 

 anterior arm of the cardinal. The pallial sinus, as well as the 

 hinge of the right valve is unknown. The dimensions of the type 

 are : height, 54.5 mm., convexity, 17.5 mm. Those of the cotype 

 are : length, 60 mm., height, 50 mm., convexity, 13 mm. 



This variety is closely related to 8. albaria, being perhaps the 

 progenitor of that species ; it varies considerably as regards general 

 shape ; elongate specimens are not separable upon the basis of shape 

 alone from the typical species. Since the great majority are more 

 ovate, and since the hinges of those specimens observed are different 

 from S. albaria, it appears to be worthy of varietal rank. It differs 

 from Conrad's species in that the hinge plate is relatively wider, the 

 left cardinal is heavier and the laterals are longer and more distant 

 from the cardinal arm. 



Occurrence. — Oligocene : Agasoma graviclum zone, Mount Diablo 

 region, California. 



The type of this variety was obtained from University of Cali- 

 fornia Locality 1687. It was collected by R. E. Dickerson, who 

 described the locality in the following words: "To the north of 

 the road leading to Tice Valley, west of the first "a" in "San 

 Ramon," Concord sheet, Contra Costa County, California. 



SPISULA SELBYENSIS, n. sp. 

 Plate 26, figures 2a, 2b and 3 

 Shell inequilateral, somewhat ventricose ; umbones rather promi- 

 nent, adjacent, generally situated posterior to the middle of the 

 shell; area below and anterior to the beaks strongly excavated; 

 anterior dorsal edge nearly straight, sloping regularly to the 

 broadly rounded anterior extremity; posterior dorsal edge broadly 

 rounded, meeting a slight truncation at the base; base arcuate; 

 surface marked by a varying number of incremental lines of growth ; 

 hinge plate very narrow ; chondophore deep and somewhat com- 



