282 University of California Publications in Geology [Vol. 9 



coalescent dorsally ; laminae smooth, the groove between the anterior 

 ones being very deep ; anterior lateral very short, distant from the 

 cardinal and arising from the ventral edge of the hinge plate; pos- 

 terior one also short and distant from the umbones; ventral lamina 

 well developed, dorsal ones very small. Pallia! sinus very shallow 

 and angular. The dimensions of the specimen figured in this paper 

 are : length 44 mm., height 34 mm., convexity 13 mm. 



This ventrieose and angulated species is easily separable from 

 the other known Pacific Coast Mactrinae upon the basis of the gen- 

 eral shape, which is quite constant in all of the specimens examined. 



Mactra exoleta has been reported from the Pliocene of Wash- 

 ington but the specimen figured scarcely seems determinable. 20 



This species was described from a Recent specimen. 



Occurrence. — Recent: San Diego, California, to Guayquil, Equa- 

 dor (Dall). Pleistocene: San Diego, San Pedro (Arnold) ; Newport, 

 California. 



MACTRA TRAMPASENSIS, n. sp. 

 Plate 13, figure 3; plate 16, figures 1 and 2 



Shell medium-sized, very ventrieose, trigonal to subovate, nearly 

 as high as long, and nearly equilateral ; sculptured by faint distant 

 concentric lines ; beaks prominent, incurved, distant ; posterior dorsal 

 slope somewhat concave, with an ill-defined escutcheon ; anterior 

 dorsal slope concave, marked by a well-defined impressed lunule ; 

 anterior and posterior extremities about equally rounded, ventral 

 margin broadly rounded. Hinge plate narrow ; chondophore deep, 

 nearly circular in outline, slightly overhung by the cardinal, prob- 

 ably roofed by a spur; left cardinal arms heavy; anterior arm 

 slightly longer than the posterior, reaching from the dorsal margin 

 of the shell to the ventral margin of the hinge plate ; right cardinal 

 partially destroyed; anterior arm strong, low, apparently confluent 

 with the ventral lamina ; laminae smooth ; anterior lateral confluent 

 with the anterior arm of the cardinal ; posterior lamina low, long. 

 Pallial line is unknown. The dimensions of the type specimen are : 

 length 48 mm., height 38 mm., convexity 14 mm. 



The distinctive shape readily separates this species from other 

 Pacific Coast Mactrinae. The few specimens that are known show 

 but little variation in general shape. 



20 Reagan, A. B., Kan. Acad. Sci., vol. 22, p. 212, pi. 4, figs. 42a, 42b, 1909. 



