PLEISTOCENE PECTENS. 129 



Descrijifidii. — Shell avcmging about 110 niillinicters in nltitude, sliglitly broader 

 than high, siil)eqiiivalve, .subeiiuiiateral, of niediuni thicknes^i, somewhat ventrico.se, 

 and with margins tinely serrate; base regularl\' rounded; sides sloping and nearly 

 straight. Right valve slightly less ventricose than left, with about 9 or 10 broad, 

 flat, subequal ribs; interspaces subequal, with slightly rounded bottoms; whole 

 surface sculptured by numerous (averaging about 7 on each large ril)) radiating 

 riblets, whose surfaces and interspaces are crossed by numerous fine, wav\^ liruhe 

 of growth; several faint undulations are also generally noticeable on the disk; 

 hinge line slightly longer than one-half length of disk; anterior ear about one and 

 three-eighths times the length of the posterior, arcuate in front, and ornamented 

 by several prominent radiating riblets and fine, sharp, incremental lines; byssal 

 notch not very prominent; posterior ear rectangularly truncated and sculptured 

 similarly to anterior. Left valve resembling the right except that it is sliglitly 

 more ventricose and the ribs are ornamented by more or less elevated, hollow 

 nodes, those on the alternating ribs being the more prominent. 



Dimensions. — Alt. 112 mm.; long. 12l> mm.; hinge line 66 mm.; diameter 

 ■4.5 mm. 



Doctor Dall is of the opinion that there is little reason for separating this 

 form from the P. ttodosus of the Antilles, although he says the latter is generally 

 thinner, and has the nodosity more pronounced. 



A fragment of a left valve of a variety of P. xuhnothmit' from the upper 

 San Pedro formation (Pleistocene) of Deadman Island, San Pedro, has 17 ribs whose 

 sides are concavely excavated, thus giving them a somewhat T-shaped cross section. 

 The middle rib and the fourth rib on each side of it are very prominent, while 

 the other ribs are much smaller. Sculpture as in typical P. suhnorldsvs. 



The specimen figured is a living shell from the western coast of Mexico, and 

 is now in the collection of the department of geology, Leland Stanford Junior 

 Universitj'. 



RANGE. 



JLiiving. Coast of Lower California to Ecuador and the tropical shores of South 



America (Carpenter). 

 Pleistocene. San Pedro (A. A. Wright; Arnold); Cerros Island and other points on 



the coast of Lowei' California (Dall). 

 Pliocene. Temescal Canyon, Santa Monica Mountains, Los Angeles County (Rivers). 



Pecten (Chlamys) latiauritus Conrad var. cerritensis u. var. 



PI. XLVI, figfi. (i anil 7. 



190o. Pecten (Chlami/s) latiauritus Conrad var. fragiUs .\rnold, Mem. Cal. .\i-ad. Sci., vol. 3, pt. 2, 1903, 

 p. 112, pi. 12, fig. 8; not Fecten fragiiis Jeffreys, Ann. and Mag. Nat. Hist., 4th ser., vol. 18, 

 1876, p. 424. 



Description. — Shell of same general type as P. latiauritus. More compressed 

 and much thinner; rilis seven to nine, very low and rounded, with shallow, rounded 

 interspaces; surface smooth, the concentric sculpture being nearlv obsolete; hinge 

 long; ears rather sharply pointed. 



17260— No. 47—06 9 



