ARNOLD -Tin; I'ALEONTOLOUV ANU STKATUiKAl'llY OV SAN I'liUUD. Ho 



[8. 1).] Pecten (Chlamys) opuntia JhdI. 



PecUn (Chlamys) opunlia Uall, Trans. Wav^ner Inst. Sci., \\)1. Ill, Part 4, 1S9S, p. 707, 

 PI. XXIX, fi^. 6. 



Shell of medium size, allied to P. htriccus var. navarclms Dall, from which il clifTers by its 

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

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

 on the coni|>letion of growth. 



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



Tlii,< species is distiiigiiisluiblo froiu llio allied species by ihe large iiiiinber 

 (forty or fifty) of iiiifixsciciilated imbricated ribs, which are of about equal prominence. 



Fomul ill the Pliocene at Pacific Beach, near San Diego, and in the Pliocene 

 at Packard's Hill, Santa Barbara. 



Pliocene. — Santti P)arl>aia (Arnold): San Diego (Hemphill; Hamlin; Arnold). 



Section ridgiocteniuin Dull. 



Sliell thin, orbicular, with subequal intlated valves, usually L(.)uilateral, with uniform, well- 

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

 frequently smooth above; the subniargins impressed below the subequal auricles; the valves well 

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



Type, Pecten veniricosus Sovverby. 



34. Pecten (Plagioctenium) newsomi, sp. nov. 



Platk XI, Fics. 1 ANU he. 



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.; diani. 8 mm.; hinge 16 mm.; angle of dorsal 

 margins 87 degrees. 



This variety is distinguishable from P. venlricosuK by the rnncli smaller angle 

 (87 degrees) made by the dorsal margins of the disk, that of a typical /'. ventricosus 

 being aiiout 110 degrees. P. neiosomi is also very much less ventricose than the 

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

 guishable from J\ snbceutricosus by much smaller angle formed by dorsal margins, 

 and by narrower ridges. It resembles J\ suhveniricosus in degree of convexity. This 

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

 University. 



Rare in upper S;in 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). 



( IB ) December 25, l'J02. 



