278 University of California Publications in Geology [Vol. 9 



shape; ligament internal or external; chondophore excavated; a 

 bifid A -shaped cardinal in the left valve fitting between the two 

 arms of the cardinal of the right valve; posterior and anterior 

 laminae well-developed ; accessory lamellae often present ; pallial line 

 often sinuated; siphons united; epidermis present. 



Subfamily MACTRINAE Dall, 1894 



Shell subequilateral ; gape slight in most forms, never pro- 

 nounced; siphons may be retracted within the shell; epidermis 

 covering the siphons not markedly developed. 



Genus Mactra Linnaeus, 1758 



Type Mactra stultora Linnaeus 

 Plate 12, figures la and lb 

 Shell ovate-trigonal, subequilateral; chondophore roofed at the 

 apex; spur distinct; ligament separated from the resilium by a 

 shelly ridge. 



Subgenus Mactrotoma Dall ; 1894 



Type Mactra fragilis Gmelin 



Shell with a depressed band on the dorsal area ; sculptured ; um- 

 bones adjacent ; gape pronounced ; resilifer large, shallow, apically 

 roofed ; cardinals prominent, but thin ; posterior arm of the cardi- 

 nals overhangs the chondophore ; accessories on the posterior arms ; 

 anterior laminae arising from the dorsal sinus ; pallial line distant ; 

 epidermis thin and silky. 



Section Mactrotoma Dall, 1894 



Mactra nasuta Gould is the first species mentioned by Dall. 

 Accessories coalescent with laterals and cardinals. 



MACTRA NASUTA Gould 

 Plate 12, figures 2a, 2b and 2c 

 Mactra nasuta Gould, Boston Soc. Nat. Hist., vol. 4, p. 88, 1851; Dall, 

 Nautilus, vol. 8, p. 39, 1894; Keep, West Coast Shells, p. 105, 1911. 

 Mactra califomica Deshayes, Proc. Zool. Soc. London, 1854, p. 68. 

 Mactra fragilis Carpenter, Brit. Assn. Rept. 1856, p. 304. 

 Mactra deshayesii Conrad, Am. Jour. Conch., vol. 3, appendix, p. 46, 1868. 



