1915] 



Clark: Fauna of the San Pablo Group 



455 



The differences in the sculpturing on the two valves is quite notice- 

 able ; the left valve, with fairly prominent rounded ribs and the sharp 

 incremental lines, contrasts quite strongly with the right valve, with 

 its low, squarish ribs and coarser incremental lines. 



Named in honor of Professor Charles E. Weaver of the University 

 of Washington. 



Dimensions.— Of left valve, plate 45, figure 2. Height, 34 mm. ; 

 length, 36 mm. 



Occurrence. — From the basal beds of the San Pablo group on San 

 Pablo Bay, University of California localities 1141 and 401. 



MYTILUS MERBIAMI, n. sp. 

 Plate 48, figures 7 and 8 



Shell thick, very ventricose. Beaks heavy, bluntly pointed. Hinge 

 plate below the beaks very heavy. Base sometimes straight, sometimes 

 gently concave. Posterior edge angulated, the point of angulation 

 being situated anterior to the middle. Posterior extremity evenly 

 rounded. Surface of shell covered by irregular coarse incremental 

 undulations. Highest point of convexity of surface anterior to the 

 middle of the valve; anterior slope high and nearly perpendicular; 

 posterior slope steep, dropping at an angle of nearly 30° as measured 

 at the point of angulation on the posterior edge ; to the rear of this 

 point the shell is not so convex ; no teeth on thickened hinge plate. 

 Thickened area below the beaks obscurely grooved ; deep elongate an- 

 terior adductor muscle scar just below the thickened hinge plate. 



M. merriami, n. sp., differs from M. coalingensis Arnold and the 

 variety of that species from the San Pablo, in that the slopes are more 

 ventricose ; the posterior slope is steeper ; the valves do not flatten 

 out toward the base so markedly; it lacks the deep pit which lies im- 

 mediately below the cardinal hinge plate on the other two forms. 



M. merriami, n. sp., differs from M. perrini, n. sp., also from the 

 Upper San Pablo Group, in being larger, in having a heavier hinge 

 plate ; the valves are higher and more convex and the anterior and 

 dorsal slopes are much steeper. 



M. merriami, n. sp., differs from M. mathewsonii Gabb, a Lower 

 Miocene form, as follows: On M. merriami, n. sp., the base is straight, 

 on M. matliewsonii Gabb it is curved, on the former the highest point 

 of convexity of the surface is nearer the median line than on the 

 latter; also the posterior extremity is not so broad. 



