106 CALIFORNIA ACADEMY OF SCIENCES. 



or seventeen very prominent, rather narrow radiating ribs, which are nearly fiat-topped and have 

 nearly perpendicular sides; interspaces wider than ribs, with rounding bottoms; fine incremental 

 lamellae cover the surface of this valve; ears square-cornered and concave, covered with incremental 

 lamelke. Lower (right) valve convex, most apparent between middle of valve and umbo; sculptured 

 by sixteen prominent squarish ribs, which are similar to those on the upper valve, except that they 

 are slightly broader and have a more rounded top; whole surface sculptured by very fine incremental 

 lines; ears similar to those of upper valve except convex, and the right one having three or four radi- 

 ating ridges and a small byssal notch. 



Dimerisions. — Long. 63 mm.; alt. 56 mm.; diam. 15 mm.; hinge-line 28 mm. 



This species is readily distinguishable from P. stearnsii and P. diegensis by its 

 smaller size, greater convexity of lower valve, and fewer ribs. Distinguishable from 

 P. bellus by smaller size, flat or concave upper valve, and narrow, more elevated and 

 prominent radiating ribs. Common in the upj^er horizon of the Pliocene at Pacific 

 Beach, and also in the strata exposed on Tenth Street, near Russ School, San Diego. 



Pliocene. — Pacific Beach and Buss School, San Diego (Hemphill; Dall; 

 Arnold). 



24. Pecten (Pecten) stearnsii Dull. 



Plate XII, Fin. 3. 



Pedoi stearyisii Dall., Proc. U. S. Nat. Mus., Vol. I, 1878, p. 14; Trans. Wagner Inst. Sci., Vol. 



Ill, Part 4, 1898, p. 706, PI. XXVI, fig. 5. 

 /antra dentata Sby. (?), Gabb, Pal. Cal., Vol. II, 1S69, p. 104 (in part). Cooper, 7th Ann. Rept. 



Cal. St. Min., 1888, p. 244 (in part). 



Shell of medium size, subcircular, inequivalve, concavo-convex, thin; right valve slightly 

 convex, with about twenty-six regular, even, square ribs, separated by channeled interspaces some- 

 what narrower than the ribs ; the top surface of each rib is flattened with a broad, shallow groove in 

 the middle, with one or two faint riblets on each side of the groove; whole surface covered with 

 concentric lamelte which are much finer and about twice as crowded as those of the left valve; left 

 valve flattened or concave, with about twenty-four regularly rounded, vaulted, even ribs, separated 

 by slightly wider channeled interspaces; the whole surface covered with fine, sharp, concentric, 

 regular lamellae, a little looped backward over the tops of the ridge; ears of this valve concave, with 

 obsolete radiating ridges, and fine, concentric lamellae; ears of right valve subequal, arched, covered 

 with crowded, elevated lamellae; byssal notch very small. 



Dimensions. — Long. 71 mm.; alt. 62 mm.; diam. 14 mm ; hinge 25 mm. 



"This is the Pliocene precursor of P. diegensis Dall (Plate XII, fig. 5), {P. 

 floridus of Hinds, not of Gmelin) from which it differs by having five or six more 

 ribs, which, in the adult, have a conspicuous median sulcus." (Dall.) Mrs. Old- 

 royd has eight P. diegensis which have been hauled up in the fishermen's nets at San 

 Pedro; two of these are over six inches in diameter and are exceptionally high 

 colored for this species. 



Two imperfect left valves from the Pliocene of Deadman Island. The figure 

 is of a left valve from the Pliocene of San Diego. Specimens identified by Dr. Dall. 



Pliocene. — San Pedro (Arnold) : San Diego (Pacific Beach, lower horizon), 

 (Dall; Hamlin; Hemphill; Arnold). 



