Geological Papers. 213 



44. Mactra (Spisula) falcata Gould. Plate V, fig. 44. 



Mactra falcata Gld., Proc. Bost. Soc. Nat. Hist., vol. Ill, 1850, p. 216; 



Wilkes's Expl. Exped., vol XII, p. 393, fig. 506, 1852. 

 Standella falcata Gld., Carpenter, Brit. Assn. Rept. 1863, p. 640; Gabb, 



Pal. Cal. vol. II, p. 92, 1869. 

 Standella nasuta Gld., Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 266; 



Keep, West Coast Shells, p. 88, 1892. 

 Mactra planulata var. falcata Gld., Williamson, Proc. U. S. Nat. Mus., vol. 



XV, 1892, p. 187. 

 Mactra (Spisula) falcata Gould., Arnold, Mem. Cal. Acad. Sci., vol. Ill, pp. 



176, 177, pi. XIX, fig. 1. 



Arnold's description:'"' "Shell of medium size, transversely 

 ovate-triangular, convex, rather thin; umbones about central, ele- 

 vated; anterior dorsal margin nearly straight; anterior extremity 

 rounded near the base; ventral margin subarcuate; posterior dorsal 

 margin slightly arcuate, and in left valve forming a slight angle 

 behind the umbo; posterior extremity not quite so sharply rounded' 

 as anterior extremity; submarginal posterior ridge rounded, not 

 prominent; hinge as in M. catiliforniis, except cartilage-pit less 

 prominent; pallial sinus rather long, with rounded extremity hori- 

 zontal; two broad flat ridges radiate from umbo on interior sur- 

 face, forming a long, triangular depression between them." 



Dimensions: Some of the older specimens are more than 100 mm. 

 in length and 80 mm. in altitude. A medium specimen measures: 

 Length, 82; alt., 66; diam., 32. A smaller specimen measures: 

 Length, 12; alt., 6; diam., 6. 



"Distinguishable from other species by narrower, more acutely 

 rounded anterior end and interior radiating ridges. Resembles 

 J/, californica, but is distinguishable by smaller pallial sinus, lack 

 of ridge between cartilage-pit and anterior ligamental groove, and 

 by the interior radiating ridges and narrow anterior end." 



This species is abundant in the Pliocene of Quillayute, Wash. 

 A single specimen was also obtained from the upper series of the 

 Oligocene-Miocene at Gettysburg, Wash. 



Living: San Diego, Cal., to Kodiak, Alaska (Cooper). 



Pleistocene: San Deigo (Cooper; Arnold); San Pgdro, Cal. 

 (Arnold). 



Pliocene: Kirker's Pass, Contra Costa county, Eagle Prairie, 

 Humboldt county, California (Cooper); Pliocene, Quillayute, 

 Wash. (Reagan). 



Miocene : Martinez, Sunol, Alameda county ; Santiago, Los 

 Angeles county; Foxin's, Santa Barbara county: Half Moon Bay, 

 San Mateo county ; Grriswold's, San Benito county ; Siebeck's, 

 Santa Clara county, California (Cooper). 



Oligocene-Miocene: Gettysburg, Wash. (Reagan). 



60. Loc. cit. 



