174 CALIFORNIA ACADEMY OF SCIENCES. 



Superfamily MACTRACEA. 



Family XXXV. MACTRID.E. 



Subfamily MAGTRIN.E. 



Genus Mactra Linne. 



Shell nearly equilateral; anterior hinge-tooth A-shaped, with sometimes a small laminar 

 tooth close to it; lateral tooth douisled in the right valve; ligament set off by a shelly lamina rising 

 between chondrophore and ligament; cardinals generally coalescent above; laterals smooth or finely 

 granular. 



Type, Mactra stultorum Linne. 



124. Mactra calif ornica Conrad. 



Plate XIX, Fio. 2. 



Mactra californica Con., Jour. Phil. Acad. Sci., Vol. VII, 1837, p. 240, PL XVIII, fig 12. Will- 

 iamson, Proc. U. S. Nat. Mus. , Vol. XV, 1892, p. 187. Dall, Trans. Wagner Inst. 

 Sci., Vol. Ill, Part 4, 1898, p. 876. 



Standella calif ornica CoN., Carpenter, Brit. Assn. Rept., 1863, p. 640. Cooper, 7th Ann. Rept. 

 Cal. St. Min., 1888, p. 265. Keep, West Coast Shells, p. 190, 1892. 



Shell of medium size, subtrigonal, subequilateral, compressed, thick; umbones subcentral, 

 turned anteriorly, margin only slightly arcuate; posterior side with a narrow, submarginal fold; 

 posterior extremity quite sharply rounded; anterior dorsal line slightly depressed in front of umbones; 

 anterior extremity evenly rounded; ventral margin evenly arcuate; surface sculptured with fine 

 concentric lines; cartilage pit triangular, deep, separated from posterior ligamental groove by a ridge; 

 cardinal teeth sharp, narrow, prominent; anterior ligamental groove long, narrow, with prominent 

 interior margin; pallial sinus rather deep, rounded, not coinciding with ventral line. 



Dime7isions. — Long. 38 mm.; alt. 26 mm.; diam. 14 mm.; umbones to anterior extremity 

 17 mm.; to posterior extremity 21 mm. 



The shell described was a small left valve, the only one so far obtained in the 

 San Pedro Pleistocene. This species closely resembles Spisula falcaia, bnt may be 

 distinguished from that species by its thicker shell, more obtusely rounded anterior 

 end, which gives it a less trigonal outline, and by the ridge which separates the 

 posterior ligamental groove from the cartilage pit. Distinguishable from other species 

 occurring in these beds by its almost central umbo, which turns slightly toward the 

 front. Several specimens of the young of this species show prominent concentric 

 undulations, reminding one somewhat of a small R(eta undulata. 



Specimens identified by Dr. Dall. (Cooper makes M. californica and M. 

 lilanulata synonymous, and under this heading reports the species from several 

 localities in the Pleistocene, Pliocene and Miocene; but until his two forms are 

 differentiated it would be useless to give his localities.) 



Rare in upper San Pedro series of San Pedro; one specimen found, which is 

 figured, and is now in the collection of Delos Arnold. Common in the Pleistocene 

 at Twenty-sixth Street, San Diego. 



Living.- — San Francisco to San Diego (C^ar])enter). 



Pleistocene.— ^a.u Pedro; San Diego (Arnold). 



