1915] 



Clark: Fauna of the San Pablo Group 



463 



the valve ; this ridge gives a subtruneate effect to the posterior ex- 

 tremity. Anterior dorsal margin only slightly depressed. Surface 

 smooth except for lines of growth. Three fairly heavy cardinals in 

 right valve; anterior cardinal thin, set close to anterior, dorsal edge. 

 Anterior cardinal of left valve heavy, not in line with the strong an- 

 terior lateral; middle cardinal bifid; posterior cardinal long, heavy, 

 separated from the heavy nymph plate by a deep groove. 



Tivela diabloensis differs from Tivela gabbi, n. sp., from the Upper 

 San Pablo, in being longer in proportion to the height. The posterior 

 dorsal margin is much more strongly depressed ; the cardinals are not 

 so heavy. It also differs in the presence of the carina or ridge along 

 the posterior margin, which gives the shell a subtruneate effect, not 

 seen on T. gabbi. T. diabloensis resembles in outline T. stultorum 

 Mawe, a recent species on the "West Coast, more than it does T. gabbi. 

 The hinge, however, is very different. It differs also in the presence 

 of the well-defined carina along the posterior margin and in the 

 character of the anterior dorsal margin. 



The writer has no material representing the species Tivela inezana, 

 a species described by Conrad from the Lower Miocene of the San 

 Inez Mountains. A comparison of the two San Pablo species with 

 Conrad 's meagre description and the figures appears to show that they 

 are distinct specifically. They differ especially in outline and sculp- 

 turing. 



Dimensions. — Type specimen (pi. 55, fig. 1), length, about 91 mm.; 

 height, 61 mm. ; diameter of one valve, 21 mm. Dimensions of co- 

 type (pi. 54, figs. 5 and 6), length, 60 mm.; height, 45 mm.; diameter 

 of valve, 13 mm. 



Occurrence. — In the Lower San Pablo Group, University of Cali- 

 fornia localities 1628, 1496, 1273, 1275, 1196, 1152, etc. 



TIVELA (PACHYDESMA) DIABLOENSIS var. ANGULATUM, n. var. 

 Plate 56, figure 1 



This form differs from T. diabloensis, n. sp., which is found in the 

 same horizon, in that it is higher in proportion to the length; it is 

 more equilateral ; the posterior dorsal margin is more strongly de- 

 pressed and the carina separating the depressed area from the re- 

 maining surface of the shell is stronger. Otherwise the two forms are 

 quite similar. 



