1916] Packard: Mesozoic and Cenozoic Mactrinae 



281 



This species was described from a Recent specimen obtained from 

 near Santa Barbara, California. 



Occurrence. — Recent : Straits of Juan de Fuca to Central America 

 (Dall). Pleistocene: San Diego and San Pedro (Arnold). 



Subgenus Mactrella Gray, 1853 



Type Mactra alata Spengler 



Shell thin, ventricose, trigonal, with a ridge marking off the 

 posterior dorsal area ; umbones prominent ; dental armature concen- 

 trated ; normal, narrow ligament ; anterior laminae very short. 



MACTRA EXOLETA Gray 

 Plate 12, figures 4a and 4b; plate 13, figure 2 

 Mactra exoleta Gray, Mag. Nat. Hist. 1837, p. 372; Carpenter, Proc. Zool. 

 Soc, London, pt. 24, p. 200, 1856; Brit. Assn. Repts. 1856, p. 246; Dall, ' 

 Nautilus, vol. 8, p. 40, 1894; Arnold, Mem. Gal. Acad. Sci., vol. 3, p. 175, pi. 19, 

 fig. 4, 1903. 



Lutraria ventricosa Gould, Boston Jour. Nat. Sci., vol. 4, p. 88, 1851. 

 Mulinia ventricosa A. Adams, Annals Nat. Hist. N. Y., vol. 5, p. 517, 1852. 

 Mactrella exoleta Carpenter, Brit. Assn. Repts. 1863, p. 543. 



Shell subtrigonal, very ventricose ; beaks prominent, slightly 

 anterior to the middle of the shell ; anterior dorsal edge straight ; 

 anterior end very broad and regularly rounded ; posterior dorsal 

 edge convex, sloping abruptly to the truncated posterior extremity ; 

 base broadly arcuate ; a prominent umbonal ridge extends from the 

 beaks to the posterior extremity ; the area posterior to this line is 

 nearly flat. Surface smooth or slightly roughened by lines of 

 growth. 



Hinge plate rather narrow ; chondophore deep, apically roofed, 

 divided into two areas by a well-developed shelly ridge charac- 

 teristic of the genus Mactra, overhung by an accessory lamella, which 

 arises from the posterior arm of the left cardinal ; left cardinal 

 prominent ; posterior arm slightly shorter than the anterior ; cardinal 

 reaching ventrally from the dorsal margin of the shell to the ventral 

 margin of the hinge plate ; right cardinal with a very heavy anterior 

 arm, which is appressed to the shell margin and extends about three- 

 fourths of the distance to the ventral margin of the hinge plate ; 

 posterior arm thin and slightly overhanging the resilifer ; arms not 



