no. 1617. NEW CRETACEOUS AND TERTIARY FOSSILS—ARNOLD. - 381 
than the latter; its concentric sculpture is also more prominent than 
that in either of the recent species. Owing to the distortion of all the 
specimens of V. montereyana it is impossible to say with certainty 
just what its original outline was, but it 1s believed to have been about 
as shown in the figured type. 
Type.—Cat. No. 165464, U.S.N.M. 
Horizon.—Monterey shale, middle Miocene. 
Locality —Santa Cruz quadrangle, Santa Cruz County, locality 
No. 121, on Newell Creek, 14 miles north of its confluence with San 
Lorenzo River. (J. F. Newsom, R. Arnold.) 
Genus MACTRA Linneus. 
MACTRA MONTEREYANA, new species. 
Plate XXXV, fig. 2. 
Description. Shell attaining a length of at least 30 mm., width a 
little more than one-half length, subtrigonal, compressed, subequi- 
valve, inequilateral; umbones a little behind middle, small, turned 
shehtly forward; anterior margin considerably longer than poste- 
rior; very gently convex; anterior extremity quite sharply angulated 
below; a faintly developed carina or angle, most prominent near 
umbo, extends from the latter to the anterior extremity ; base only very 
gently rounded ; posterior dorsal margin nearly straight, sloping only 
moderately steeply, posterior extremity regularly rounded and situ- 
ated nearly midway between base and umbo; surface sculptured by 
numerous fine incremental lines and numerous obsolete short undu-. 
lations. 
~Dimensions.—Leneth, 31 mm.; latitude, 18 mm.; diameter single 
valve, 2+ mm.; angle between dorsal margins, 130°. 
Notes.—This species may be distinguished from all others of the 
same genus from the west coast by its great relative length and long, 
attenuated anterior extremity. It is very closely allied to M7. dolabri- 
formis Conrad of the recent Lower California fauna, but may be dis- 
tinguished from the latter by its relatively greater length, less steeply 
sloping posterior dorsal margin, more central posterior extremity, 
and more attenuated anterior end. Considering the fact that all of 
the specimens of J/. montereyana so far examined have been sub- 
jected to at least sheht deformation by stresses within the containing 
shales, the separation of J/. montereyana from M. dolabriformis is 
attended with some uncertainty. 
Type.—Cat. No. 1654638, U.S.N.M. © 
fHorizon.—Monterey shale, middle Miocene. 
Locality—Santa Cruz quadrangle, Santa Cruz County, locality 
No. 122, on Love Creek, 1 mile above its confluence with San Lorenzo 
River. (J. F. Newsom, R. Arnold.) 
