ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 105 



Sliell of medium size, subcircular, inequivalve, concavo-convex, rather thin; right valve gib- 

 bous, with twenty smooth, rounded, only moderately convex ribs, separated by very faintly channeled, 

 narrow interspaces; surface smooth except for fine, undulating lines of growth; ears on this valve con- 

 vexly bent, and separated from the disk by a deep groove; anterior ear with small byssal notch, 

 obsoletely, radiately ribbed, and with fine incremental lines; posterior ear similar except that it lacks 

 the notch; left valve concave, with nineteen or twenty prominent squarish ribs, which are separated 

 by deep channeled interspaces of equal width with the ribs; a faint rounded rib runs along the middle 

 of each of the interspaces; surface with fine undulating incremental lines; ears of this valve concave, 

 with obsolete radiating ridges and fine lines of growth. 



Dimensions. — Long. 70 mm.; alt. 67 mm.; diam. 25 mm.; hinge 35 mm. 



Distinguishable from P. hemphiUl Dall by greater convexity of the riglit 

 valve, and by the greater number of ribs, P. hemjjhilll having only about fifteen ribs; 

 distinguishable from P. stearnsii and /-■. diegensis Dall by greater convexity of right 

 valve, low rounded ribs rather than square ones on this valve; and by square ribs, 

 between which are faint rounded riblets, rather than sharply rounded ribs on the left 

 valve. P. dentatns is of the same shape as the Japanese species F. laqueaius Sby., 

 which has fewer, but squarer, broader ribs on the right valve. This last species has 

 been reported from the Tertiary of Japan by Dr. Brauns in his Geology of the 

 Environs of Tokio.' "P. laqueatus has been erroneously cited by Reeve from Cali- 

 fornia" (Dall). 



An almost perfect right valve of this beautiful southern shell was found in the 

 upi)er San Pedro series of Ban Pedro. It differs from a living specimen from the 

 Gulf of California by having twenty rather than twenty-three ribs, and by having 

 slightly less convex ribs and wider interspaces. In degree of convexity and in its 

 ears it agrees exactly with the living shell. The specimen was identified by Dr. Dall. 



Cooper has mistaken the flat valve of P. stearnsii for this species. A specimen 

 in [he State Museum collection at the University of California, Berkeley, labeled 

 "Janira dentata Sby., San Pedro, Quaternary," is a P. stearnsii from the Pliocene of 

 Dead man Island. 



The specimen figured is a living shell from the Gulf of California, and is now 

 in the collection of the Department of Geology, Leland Stanford Junior University. 



Living. — Gulf of California (Carpenter; Button). 



Pleistocene. — Santa Barbara to San Diego (Cooper): San Pedro (Arnold). 



Pliocene. — San Diego well (Cooper) — (probably P. hemphilli). 



[S. D.] Pecten (Pecten) hemphilli Dall. 



Peden hemphilli Dall., Proc. U. S. Nat. Mus., Vol. I, 1879, p. 15. Cooper, 7th Ann. Rept. Cal. 

 St. Min., 1888, p. 257. Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1S9S, p. 706. Not 

 Jaiiira bella CoN., Pac. R. R. Rept, Vol. VI, 1857, p. 71, PI. Ill, fig. 16; nor Gabb, Pal. 

 Cal., Vol. II, 1869, PI. XVI, fig. 20. 



Shell of medium size, thin, inequivalve. Upper (left) valve flat or slightly concave, with a 

 concave depression between middle of valve and umbo; surface of this valve sculptured by sixteen 



> Mem. Duiv. Tokio, No. 4, 1881, p. J8, PI. VII, fig. 31. 

 , n I November 3, 1802. 



