108 CAXIFOKNIA ACADEMY OF SCIENCES. 



[S. D.] Pecten (Patinopecten) expansus Dull. 



Pecten expaiisus Dall, Proc. U. S. Nat. Mus., Vol. I, 1879, p. 14; Trans. Wagner Inst. Sci., Vol- 

 III, Part 4, 1898, p. 706. 



Shell large, thin, slightly convex ; outer surface of upper (left) valve marked by sixteen to 

 twenty sharp radiating ridges, but slightly elevated, and whose sides shade off insensibly into the 

 broad interspaces ; faint indications of ridges appear between the principal ones ; surface of upper 

 valve covered with fine, slightly raised, sharp lamelke ; lower (right) valve with twenty or thirty 

 dichotomous ribs, flattened above, but not sharply differentiated from the interspaces, sculptured with 

 fine lines of growth, with faint appearances of radiating striae ; ears rather small, and distinctly but 

 not strongly marked off from the disk ; byssal notch rounded, moderately deep. 



Dimensions. — Long. 140 mm.; alt. 135 mm.; diam. 32 mm.; hinge-line 65 mm. 



TIiLs species is close to P. cnuvlnus, but may be distinguished by the dicho- 

 tomous ribs on the lower valve, and the faint ribs between the principal ones on the 

 upper. A species of Pecten near, if not identical, to P. expansus is found in the Plio- 

 cene near Purisima, San Mateo County, California. P. expans^is is common in the 

 lower horizon of the Pliocene at Pacific Beach, near San Diego. 



Pliocene. — Pacific Beach, San Diego (Hemphill; Dall; Hamlin; Arnold). 



Section Nodipecten Dall. 



Both valves convex, usually of large size and heavy; ribs intermittently nodose, with 

 more or less prominent hollow nodes or buttae ; radial striation pronounced ; ears unequal, the pos- 

 terior smaller; the valves often more or less oblique; imbricate surface layer sometimes very marked. 



Type, Pecten nodosus Linne. 



26. Pecten (Nodipecten) subnodosus Sowerby. 



Pecten subnodosus Sby., Proc. Zool. Soc, 1835, p. 109. Carpenter, Brit. Assn. Rept., 1856, p. 311; 

 Brit. Assn. Rept, 1863, p. 621. Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, iS98,p. 710. 

 Pecten intermedius Con., Am. Jour. Conch., Vol. Ill, 1867, p. 7. 



Shell large, suboval, convex; surface sculptured with numerous high, square ribs, three or 

 four of which are generally more prominent than the others; interspaces deeply channeled, averaging 

 about equal in width with the ribs; surface, including the ribs and interspaces, sculptured by fine 

 radiating, squamose ridges, and fine lines of growth; ears rather small, sculptured in same manner 

 as the disk. 



Dimensions. — Long. 120 mm.; alt. 125 mm.; diam. 45 mm. 



This large tropical species is distingui.shable by strong elevated ribs and pecu- 

 liar squamose, radiating ridges. Dr. Dall says of this species: "There seems to be 

 little reason for separating this form from the P. nodosus of the Antilles. Both vary 

 through a strictly analogous series of mutations." 



One-half of a large left valve was found in the upper Sau Pedro conglomerate 

 of Deadman Island by Dr. A. A. Wright, who also found a young specimen of the 

 same species in the upper San Pedro series of San Pedro. 



Living. — Lower California to Panama (Carpenter). 



Pleistocene. — San Pedro; San Diego (Arnold): Cerros Island and Lower Cali- 

 fornia (Dall). 



