1915] 



Clark: Fauna of the San Pablo Group 



475 



MACOMA DIABLOENSIS, n. sp. 

 Plate 61, figures 8, 9, and 10 



Shell medium in size, subtrigonal in outline; beaks low, nearly 

 central. Left valve slightly more convex than right. Anterior dorsal 

 edge slightly arcuate ; posterior dorsal edge straight ; posterior ex- 

 tremity bluntly pointed; anterior end evenly rounded; ventral edge 

 gently and evenly arcuate. Right valve flexed near the posterior ex- 

 tremity; posterior dorsal margin strongly depressed, the narrow de- 

 pressed area being nearly at right angles to the main surface of the 

 valve and separated from it by a well-defined line ; on this narrow, 

 depressed area is a faint groove which does not reach to the beak. 

 Left valve near the posterior extremity bent to the right ; posterior 

 margin not depressed. Surface of shell smooth, except for more or 

 less prominent concentric lines of growth. Ligamental groove long, 

 of the M. nasuta type; interior unknown. 



M. diabloensis differs quite radically in outline from M. nasuta 

 Conrad, a Recent species ; the beaks are more central ; the posterior 

 extremity is not truncated and the depressed dorsal margins are very 

 different. 



M. diabloensis resembles quite closely in outline M. astori Dall 

 ("Miocene of Astoria and Coos Bay, Oregon," U. S. G. S. Prof. Paper, 

 no. 59, pi. 14, figs. 1, 11), but differs from it in the character of the 

 depressed posterior dorsal margin ; also the shell is not so large. 



For a comparison of M. diabloensis with M. andersoni, n. sp., found 

 in the same horizon, see the description of the latter. 



M. diabloensis differs from M. calcarea Gmelin in the character of 

 the posterior dorsal margin, and in that the beaks are more central. 



Dimensions. — Type specimen : left valve, height, 22 mm. ; length, 

 27 mm. ; diameter, 3 mm. Co-type, right valve : height, 21 mm. ; length, 

 29 mm. ; diameter, 2 mm. 



Occurrence. — A common species in the Upper San Pablo Group on 

 the south side of the Mount Diablo anticline, University of California 

 localities 763, 1513, etc. 



MACOMA PABLOENSIS, n. sp. 

 Plate 61, figures 1, 2, and 17 

 Shell small, sub-equilateral, subtrigonal in outline ; beaks incon- 

 spicuous and central. Anterior dorsal edge very slightly arcuate ; 

 posterior dorsal edge straight; ventral edge regularly and rather 



