3112 



University of California Publications in Geology 1 v °l- 9 



This species was described by Arnold as follows : 



Shell averaging from 40 to 50 mm. in length, subtrigonal in outline, alti- 

 tude about five-sevenths of length, equivalve, equilateral, the beaks located 

 midway, the shell moderately thin, moderately convex. Anterior margin only 

 very slightly depressed in front of beaks; quite evenly but somewhat acutely 

 rounded in front; base evenly rounded; posterior extremity not quite as 

 attenuated as anterior; a faint suggestion of an angle or carina extends from 

 the beak to the posterior extremity. Surface sculptured by numerous small 

 but sharply defined quite regular concentric ridges, separated by interspaces 

 somewhat narrower than the latter, also by fine incremental lines which 

 appear more prominent where the concentric ridges are obsolete. Hinge only 

 moderately strong, umbonal pit small; lateral teeth high and thin, extending 

 above the adjacent margins of the shell. Interior unKnown. The dimensions 

 of the type are: length, 50 mm., height, 34 mm., convexity, 12 mm. 



This species has not been available for study, therefore its defi- 

 nite relationships are unknown. 



The type of this species was obtained from the Chico series at 

 Pigeon Point, San Mateo County, California. It was deposited in 

 the United States National Museum under catalogue number 31001. 



Occurrence. — Cretaceous : Chico series, Pigeon Point, San Mateo 

 County, California (Arnold). 



MACTRAC?) TENUISSIMA Gabb 

 Mactra tenuissima Gabb, Pal. Cal., vol. 2, p. 179, pi. 29, fig. 68, 1869. 



Gabb described this species as follows : 



Shell small, extremely thin, subequilateral; beaks central, small, pointed 

 slightly in advance; anterior end obliquely and convexly subtruncated; base 

 regularly convex. Surface marked by minute, concentric striae of growth; an 

 angulated ridge runs from the umbones to the posterior basal angle. Pallial 

 sinus deep and narrow. The dimensions of the type as given by Gabb are 

 equivalent to: length, 27.9 mm., height, 19.3 mm., and convexity, 3.8 mm. 



This form has been reported from the Martinez group of Lake 

 County by Dickerson.-" The specimens upon which his determina- 

 tions are based do not reveal the generic characters of the form. It 

 must therefore be listed as a questionable mactrine species. 



Occurrence. — Eocene: Martinez group (Dickerson). 



26 Dickerson, R. E., Univ. Calif. Publ. Bull. Dept. Geol., vol 8, p. 96, 1914. 



