122 



University of California Publications in Geology [Vol.11 



the type locality ; the other comes from the north side of Mount 

 Diablo, University of California locality 7 6. 



A. shumardi is closely related to A. muta, n. sp. ; for the compari- 

 son of these two species see the description of the latter. The 

 specimen figured came from the type locality on the banks of the 

 Nehalem River at the town of Pittsburg. 



Genus NUCULA Lamarck 



NUCULA POSTANGULATA, n. sp. 

 Plate 13, figures 2-5 

 Type specimen 11260, Univ. Calif. Coll. Invert. Palae., loc. 2754 



Shell small, equivalved, inequilateral ; subtrigonal to subovate in outline ; beaks 

 rather inconspicuous, opisthogyrous. Posterior dorsal slope abrupt, being a little 

 more than at right angles to the anterior dorsal slope. Anterior end regularly 

 rounded ; posterior end subangulate. Escutcheon depressed and well-defined, 

 lanceolate to subcordate, extending nearly to the posterior end, pouting rather 

 sharply along its entire length. Lunule absent. Surface smooth except for fine, 

 somewhat irregular incremental lines. Hinge plate unknown. 



Dimensions. — Type specimen: length, 6.25 mm.; height, 4.75 mm. 



Occurrence. — University of California localities 1131 and 2754. 



NUCULA BIFIDA, n. sp. 

 Plate 12, figures 10, 11 and 12 

 Type specimen 11184, Coll. Invert. Palae. Univ. Calif., loc. 331 



Shell small, subtrigonal in outline ; beaks inconspicuous, recurved. Anterior 

 dorsal slope gently convex; posterior dorsal slope very steep; apical angle a little 

 more than 90°. Anterior end broadly and regularly rounded; ventral edge 

 rather strongly and regularly arcuate. Surface of type sculptured by 22 fairly 

 heavy, rounded, radiating ribs, a few of which are bifid or split near their lower 

 ends ; interspaces between the radiating ribs about equal to the width of the ribs. 

 Escutcheon only very slightly depressed, but marked off by oblique radiating ribs 

 which branch off from the last rib of the general surface ; some of these are bifid 

 distally. Lunule not depressed but marked by a sculpturing similar to that of 

 the escutcheon. Hinge plate unknown. 



Dimensions. — Length, 6 mm. ; height, 4.5 mm. ; greatest diameter of both 

 valves, 3 mm. 



Occurrence. — University of California locality 331. 



This beautiful species appears to be quite unique ; the writer has 

 found nothing on the "West Coast, either fossil or recent, with which 

 to compare it. Though the hinge plate is not known, yet the general 

 outline and sculpturing of the shell would seem to place it under 

 the genus Nucula. 



