ARNOLD -THE PALEONTOLOGY AND STRATIOKAPHY OF SAN PEDRO. 113 



[S. D.] Pecten (Chlamys) opuntia iJail. 



Pec/en ( Chlamys) ofmntia Dai.l, Trans. Wagner ln.st. Sci., Vol. Ill, Part 4, 1898, p. 707, 

 PI. XXIX, ti.t;. 6. 



Sliell of medium .size, allied to P. hcriceiis var. navarchiis Dall, from which it differs by its 

 smaller and not fasciculated radial ribs, more elongated anterior ear, more densely, radially costate 

 posterior ear, small size when adult, and by a tendency to be suddenly contracted at the basal margin 

 on the completion of growth. 



Dimensions. — Long. 32.5 mm.; alt. 35 mm. 



This species is distinguishable from tlie aliiecl species by the large luiraber 

 (forty or fifty) of iin fasciculated iinbricuted ribs, which are of about equal prominence. 



Found in the Pliocene at Pacific Beach, near San Diego, and in the Pliocene 

 at Packard's Hill, iSanta Barbara. 



Piiocene. — Santa Barbai'a (Arnold): San JJiego (Hemphill; Hamlin; Arnold). 



Section PU((jwcteati> III Dull. 



Shell thin, orbicular, with subequal inflated valves, usually equilateral, with uniform, well- 

 marked radial, not dichotomous ribs; the concentric sculpture in looped lamella-; the ribs strong, 

 frequently smooth above; the subm;irgins impressed below the subequal auricles; the valves well 

 inflated, with a tendency to oblique growth in the adult. 



Type, Pecten venlricosus Sowerby. 



34. Pecten (Plagioctenium) newsomi, sp. no v. 



Plate XI, Figs. 1 and Irt. 



Shell of medium size, ovate-triangular, compressed, equilateral, rather thin; twenty-two 

 stout, smooth-topped, squarish ribs, with channeled interspaces of same width as ribs ; interspaces 

 and sides of ribs showing sharp imbricating lines of growth; ears of left valve subequal, with subacute 

 corners; surface of ears radially striated, and showing sharp incremental lines. 



Dimensions. — Long. 24 mm.; alt. 26 mm.; diam. 8 mm.; hinge 16 mm.; angle of dorsal 

 margins 87 degrees. 



This variety is distinguishable from P. venlricosiiK by the much smaller angle 

 (87 degrees) made by the dor.saI margins of the disk, that of a typical P. veniricosus 

 being about 110 degrees. P. newsomi is also very much less ventricose than the 

 latter, and has much narrower ridges, wider interspaces, and a thinner shell; distin- 

 guishable from P. suboentricoaus by much smaller angle formed by dorsal margins, 

 and by narrower ridges. It resembles P. suhventricosus in degree of conve.xity. This 

 species is named in honor of Dr. John F. Newsom, of Leland Stanford Junior 

 University. 



Rare in upper San Pedro series of San Pedro and Los Cerritos. The speci- 

 men figured is the type, which is from the upper San Pedro series at Los Cerritos, 

 and is now in the United States National Museum. 



Pleistocene. — San Pedro (Arnold). 



( 15 ) December 26, 19011 



