1915] 



Clark: Fauna of the San Pablo Group 



457 



minute teeth may be counted. Ligamental groove broad and fairly 

 heavy ; anterior adductor scar long- and narrow. 



This species is very closely allied to M. edulus Linn. ; it differs in 

 having heavier lines of growth ; the basal margin and posterior mar- 

 gin posterior to the point of angulation are nearly parallel or slightly 

 diverging while on M. edulus they are converging; the hinge plate is 

 very much heavier and when there are teeth on the anterior margin, 

 they are finer and more numerous; the ligamental groove also appears 

 to be somewhat heavier. 



M. perrini, n. sp., differs from M. mathewsonii Gabb in that the 

 latter is a larger form ; the general outlines of the two shells are 

 different ; the highest point of convexity on the surface of M. mathew- 

 sonii Gabb is near the middle of the shell, on 71/. perrini it is in front 

 of the middle; further, the posterior slopes of M. mathewsonii Gabb 

 are steeper than on M. perrini. The writer has been unable to get a 

 good specimen of the hinge of M. mathewsonii. 



M. perrini, n. sp., resembles M. californicus Conrad in the fairly 

 coarse concentric undulations ; it lacks the heavy radial sculpture of 

 M. californicus : the hinge plate and outline are also quite different. 



M. perrini differs from M. expansus Arnold (listed by Arnold as 

 M. mathewsonii Gabb var. expansus, n. var.) in that it is a smaller 

 form ; the posterior dorsal margin is shorter ; the surface lacks the fine 

 radiating striae seen on M. expansus Arnold. In other respects the 

 two forms seem to be quite similar; both have a broad, subtruncate 

 ventral edge and usually a straight base, the anterior slope of the 

 surface being quite abrupt, with the highest point of convexity well 

 in front of the middle of the shell. 



Dimensions. — Type specimen : greatest anteroposterior diameter 

 about 57 mm. ; greatest width measured at right angles to base, 25 mm 



Occurrence. — Upper San Pablo Group on San Pablo Bay, Univer- 

 sity of California locality 1617. 



Named in honor of Professor James Perrin Smith, Professor of 

 Palaeontology at Leland Stanford Junior University. 



MYTILUS (MYTILOCONCHA) TKAMPASENSIS, n. sp. 

 Plate 42, figures 2 and 3 



Shell medium to large, low, slender ; beaks acute, not twisted ; base 

 nearly straight, on some specimens gently concave. Cardinal angle 

 near the middle of the elongate axis. Posterior end evenly rounded: 



