58 



University of California Publications in Geology [Vol. 10 



the internal riblets to reach the periphery. The external sculpture 

 of the left valve and the outline and submicroscopic radiating striae 

 of the right, described in the new species, are characters quite distinct 

 from P. alaskensis. 



Four specimens were found, of which two are nearly perfect 

 left valves, and of the two right valves, one is nearly intact. 



MACROCALLISTA DENSA, n.sp. 



Plate 2, Figures la, 1ft 



Univ. Calif. Coll. Invert. Palaeont., no. 11086. Type locality, Fourth and 

 Broadway, Los Angeles. 



Shell small, thick, broadly subovate, inequilateral, moderately con- 

 vex. The prosogyrous beaks are moderately elevated. Surfaces of 

 both valves marked by fine incremental lines which occasionally are 

 somewhat enlarged and roughened. Anterior and posterior ends evenly 

 rounded, but as the posterior is somewhat the broader, the shell appears 

 subtruncate. Posterior dorsal margin straight fom the umbo to one- 

 half the distance to the posterior end; ventral margin smooth and 

 thick. No escutcheon appears, and the lunule is very faintly marked 

 by a circumscribing line. Three prominent cardinal teeth appear 

 in each valve, the posterior being longer and narrower than the other 

 two and strongly bifid in each valve. In the right valve are two 

 prominent anterior elaspers while a corresponding lateral appears in 

 the left; the ventral lateral in the right valve is in alignment with 

 the anterior cardinal, which is shortened. The nymph plate is smooth 

 and, in the right valve, is elevated at some distance posterior to the 

 last cardinal. The external ligamental groove is long and relatively 

 broad. In the right valve the hinge plate bears a rounding embay- 

 ment ; in the left it is triangular. Pedal scars indistinct ; pallial line 

 distant from the ventral margin ; pallial sinus capacious and broadly 

 rounding. 



Dimensions. — Altitude, 7.4 mm. ; diameter, 8 mm. 



This beautiful little shell possesses unique characters in the placing 

 of the anterior laterals and in having both posterior cardinals bifid. 

 No near relatives of the form appear in the California Tertiary. 



Four specimens were found, of which one is a right valve slightly 

 broken. 



