104 rALIFOKNU ACADEMY OF SCIENCES. 



about ninety degrees; surface of right v.ilvc ornanieiUcii by fourteen ur fifteen prominent, nearly llat- 

 topped, square, radiating ribs, some of them \vitl> one or two longitudinal obsolete lines; the ribs 

 become somewhat less elevated and the sides more slojiing as the ])eri|)lKMy is apiiroached in the 

 adult; surface of right ilisk ornamented with close, fme, stjuamosc, concentric wrinkles; ears subecpial, 

 arched, covered with crowded, elevated lamella-; byssal notch small. 



Dimensions. — Alt. So mm.; lat. loS mm.; diam. .^.? mm.; length of hinge-line, 45 nun. 



F(»mul ill till' Pliocene al Simla Barltara, ("alifoniia (Conrad. (iaMi, ^'ales 

 Cooper. Arnold). 



The description, measurement.-^, and figures are of Gabb'.s type i^pecimeii of 

 Janira bi'lhi (No. !•()(). ('nllcctidii .Vcadtiny Natural Science.s, Philadelphia), which 

 was kindly loaned to the writer Ky I'lofessor H. A. Pilsbry, Curator of MolUisca. 



y. steal nsii and /'. ilki/ciisis are distinnuisliable from P. hellus by the more 

 lunneious (25 or 28 in the first, 20 or 22 in the second), narrower, sliarper defined, 

 perpendicular-sided, radiating ribs on tbe right valve, and by evenly rounded, promi- 

 nently and evenly lainellated ribs of the left valve. /'. heniphilli is distinguishable 

 from I\ hellus by its smaller size; by liaving on the right valve more numerous (l.! 

 or 10 in the former), round-topped, narrower, nearly perpendicular-sided, radiating 

 rii)s, which retain their prominence for their entire length, and by the less convexity 

 of the disk, more mimerous, narrower and more elevated nidiating rilis of the left 

 valve. P. hmnphiHi has the same depression below the apex in the lett valve, l)iit 

 the less degree of convexity of the rest of the disk lessens the prominence of the de- 

 pression, which is so marked in most specimens of P. bellas. P. deutalus is dis- 

 tinguishable from P. hellus by its smaller size, greater convexity of right valve, greater 

 concavity of left valve, greater niiml)er of ribs, and by the auxiliary ribs in the 

 left valve. 



After a careful comparison of a large series of 1'. hellus with Conrad's descrip- 

 tion and figure and Gabb's figure and t3'pe specimen, tbe writer has no hesitancy in 

 adopting tbe synonymy given at the beginning of this article. Dall is of the opinion 

 that Conrad's species and that of Gabb are diflerent. This idea was prob;ibly caused 

 by the exaggeration of the bicarination of the ribs in Conrad's figure. Several of 

 the specimens of P. bellus examined by the writer show this bicarination to a greater 

 or less degree, although as a rule the ribs are nearly smooth-topped. A large series 

 of P. hellus am\ P. hemphilli show the diflerences enumerated in a previous paragraph 

 to be constant for the adults. The young of both species up to an altitude of twenty 

 millimeters are nearly identical in appearance. 



23. Pecten (Pecten) dentatus Soiverby. 



Plate XII, Fios. 1 and ]ri. 



Pecten dentatus Sbv., Thes. Conch., Vol. I, p. 39, PI. XV, figs. 105, 106. I).\li„ Trans. Wagner 



Ins. Sci., Vol. Ill, Part 4, 1892, p. 707. 

 Vola dentata Sbv., H. &. A. Adams, Gen. Rec. Moll. 

 /antra dentata Sbv. (=/. cxcavata, Val., yfa?^ Carpenter, Brit. Assn. RejU., 1863, p. 654). Gabb, 



Pal. Cal., Vol. 11, 1869, p. 104. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 244 (— /*. 



sfearnsii Dall, in part). 



