464 University of California Publications in Geology [Vol. 8 



The writer has only one good specimen of the right valve, but it is 

 so distinct from the typical form, T. diabloensis, n. sp., that it appears 

 to deserve at least varietal recognition. 



Dimensions. — Height, 48 mm.; length, 58 mm. 



Occurrence. — University of California locality, 1623, Lower San 

 Pablo of the type section. 



TIVELA (PACHYDESMA) GABBI, n. sp. 

 Plate 55, figure 2; plate 56, figures 3 and 4 



Shell large, heavy, trigonal in outline; beaks subcentral, slightly 

 posterior to the middle. Posterior dorsal edge nearly straight, on 

 some specimens slightly concave; anterior dorsal edge slightly con- 

 vex ; anterior end regularly rounded ; posterior end regularly rounded 

 but not so broad as the anterior ; ventral edge long and gently arcuate. 

 Surface of valve moderately convex, smooth, except for medium fine, 

 more or less prominent incremental lines. Left valve with three heavy, 

 elongate cardinals ; nymph plate heavy, separated from the posterior 

 cardinal by a deep groove, which extends up to the beak ; middle 

 cardinal slightly bifid ; anterior lateral heavy, elongate, not in line 

 with the anterior cardinal. Three cardinals in right valve; the two 

 posterior cardinals are heavy and elongate ; anterior cardinal thin and 

 situated close up to the anterior dorsal margin ; here again the nymph 

 plate is heavy and may very easily be mistaken for a cardinal ; socket 

 for lateral tooth of left valve deep and heavy; pallial sinus short and 

 blunt; muscle impressions equal. Posterior dorsal edge strongly 

 grooved to receive the posterior edge of the left valve. 



T. gabbi is closely related, if not in direct line of descent, to T. 

 stultorum Mawe, a Recent species. It differs from that species in 

 that the shell is higher in proportion to the length ; the beaks are more 

 posterior ; the dorsal margins are not as strongly convex ; the hinge 

 plate is slightly heavier just above the lateral teeth ; on the left valve 

 the anterior lateral is not in line with the anterior cardinal as in 

 T. stultorum ; the nymph plate of the left valve is very prominent, 

 while on T. stultorum Mawe this is almost obsolete; the anterior 

 cardinal on T. gabbi, n. sp., is longer and heavier. A comparison of 

 the hinge plates of the right valve of the two species shows that on 

 T. gabbi it is somewhat heavier ; the nymph plate is heavier and 

 slightly arched upward while on T. stultorum Mawe it is depressed 

 inward. 



