1915] 



Clark: Fauna of the San Pablo Group 



465 



T. gabbi, n. sp., differs from T. diabloensis, n. sp., chiefly in outline; 

 the latter species is longer in proportion to the height, and the surface 

 is not so strongly depressed along the posterior dorsal margin ; Tivela 

 gabbi, n. sp., lacks the truncation on the posterior end and the obscure 

 ridge or carina extending from the peak to the posterior extremity, 

 which are seen on T. diabloensis, n. sp. 



Dimensions. — Type specimen: height, 74 mm.; length, 86 mm.; 

 diameter of one valve, 17 mm. 



Occurrence. — A very common species in the upper San Pablo 

 Group, University of California localities 1272, 1155, 1281, 1625. 1942, 

 1943, 1955, etc. 



The following description of the Section Pachydesma, under Tivela 

 is given by Dall: 48a "Shell very large, elongate — trigonal with smooth 

 interior margins, a vernicose, dehiscent periostraeum, and four car- 

 dinals in each valve. ' ' After examining a great many specimens, the 

 writer has come to the conclusion that in each valve what has been 

 taken by Dall as the posterior cardinal is rather to be considered as 

 a nymph plate. These oppose one another and do not alternate as true 

 cardinals should. Thus the dental formula of section Pacliyelesma 



. L 010101.010. Ti , , , ^ 



should be written It may be questioned whether the 



K 101010.101 



section Pachydesma should not be raised to generic rank, as the genus 

 Tivela is defined as having supplementary cardinals. 



AMIANTIS (AMIANTIS) DALL I, n. sp. 

 Plate 50, figure 2; plate 53, figures 1, 2, 3, and 4 



Shell large, heavy, inequilateral, subtrigonal to subovate in outline ; 

 height, about equal to length ; beaks prominent, strongly incurved and 

 anterior to the middle of the shell. Anterior dorsal edge gently con- 

 cave ; posterior dorsal edge long and gently convex ; ventral edge regu- 

 larly and strongly arcuate; anterior end regularly rounded; posterior 

 end subangulate. Lunule well defined, large, wider on right valve 

 than on left. Surface along the posterior dorsal margin strongly de- 

 pressed, forming an escutcheon-like area which is separated from the: 

 main outer surface of the shell by a distinct line. Surface sculptured 

 by heavy concentric ridges or undulations, between and upon which 

 are finer incremental lines; toward the beaks and on most of the 

 surface of the shell the interspaces between the ridges are consider- 



48a Wagner Free Institute of Science. Transactions, vol. Ill, pt. VI, p. 1244. 



