No, 1617. NEW CRETACEOUS AND TERTIARY FOSSILS—ARNOLD. 357% 
CRETACEOUS SPECIBS. 
PE bE CVwPoODA 
Genus MACTRA Linneceus. 
MACTRA STANTONI, new species. 
IPIB WWE SOxOr LE, Take BS 
Description.—Shell averaging from 40 to 50 mm. in length, sub- 
trigonal in outline, altitude 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 margin sloping straight back 
from umbones; posterior extremity not quite as attenuate 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 inter- 
spaces 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. 
Dimensions.—Length= (restored), 50 mm.; latitude, 34 mm.; 
diameter (1 valve), 12 mm.; umbonal angle about 120°. 
Notes.—This species 1s quite unlike any other found in the Cre- 
taceous of the west coast. Distorted specimens, however, resemble 
Tapes conradiana Gabb, owing to the peculiar appearance of the 
concentric ribs. Numerous specimens of J/. stantoni occur in the 
sandstone at locality No. 27, north of Pigeon Point, where most of 
the individuals are more or less distorted. The specimen figured 
is the best of those collected. 
Named in honor of Dr. Timothy W. Stanton, chief ee eet 
United States Geological Survey. 
Lype.—Cat. No. 31001, U.S.N.M., 1 mile north of Pigeon Point, 
localitiveNon2 (= 
[Tovizon.—Chico formation, upper Cretaceous. 
Locality. Santa Cruz aendeanile: San Mateo County, locality 
No. 27. (Arnold.) 
