1914] Dickerson : Fauna of the Martinez Eocene of California 125 



by its concave dorsal posterior margin and by its short anterior margin. 

 The knife-like form identifies it readily. 



Named in honor of Mr. J. R. Powers, who made very careful collec- 

 tions in the Martinez at Selby Smelter. 



PINNA BAKKOWSI, n. sp. 

 Plate 8, figure 3 



Description. — The shape of the shell is elongate-cuneate. The cross- 

 section of the shell is quadrate-oval. The valves meet on their dorsal 

 margins at a sharp angle which increases from sixty degrees near the 

 base to ninety degrees near the beak. The ventral portions of the 

 valves are rounded. Each valve presents two slopes : a flat, dorsal 

 and a slightly convex, ventral which meet along a radial line. This 

 line divides the shell so that the flat dorsal slope is one-third and the 

 rounded ventral, two-thirds of the total surface. The rounded ventral 

 slope is marked in its central half by approximately eight compressed 

 and slightly elevated radial ribs whose interspaces are twice their 

 width, and by eight to ten concentric rounded ribs in its marginal 

 portion as well as radial ribs, thus further dividing the convex slope 

 into equal parts. These ribs do not show on figured specimen. The 

 flat dorsal slope of the shell is ornamented by about ten compressed 

 radial ribs which are separated by interspaces two times as wide. The 

 nearly straight posterior end makes almost a right angle with the 

 ventral margin. 



Dimensions. — Total length, unknown. Length of specimen whose 

 beak and lower margin are broken, 30 mm., width, 15 mm., approxi- 

 mate. 



Occurrence. — Two and one-half miles south of Muir Station and 

 one-half mile east of the road which crosses the Briones Hills going 

 south from Muir Station, on north slope of a small tributary of a 

 tributary of Arroyo del Plambre. Elevation, 550 feet. About 100 

 yards north of Monterey-Martinez contact, University of California 

 Locality 1547, and at University of California Locality 213. 



Notes. — Plioladomya nasuta, Trochocyatlius zitteli, Urosyca cau- 

 data, Siphonalia( ?) lineata, and many other Martinez species are 

 associated with this form. Pinna barrowsi has two kinds of ornamen- 

 tation while P. brewerii has but radial ribs. The gore or plait which 

 divides both valves of P. brewerii is approximately central, while the 



