104 CALIFOENIA ACADEMY OF SCIENCES. 



about ninety degrees; surface of right valve ornamented by fourteen or fifteen prominent, nearly flat- 

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

 become somewhat less elevated and the sides more sloping as the periphery is approached in the 

 adult; surface of right disk ornamented with close, fine, squamose, concentric wrinkles; ears subequal, 

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



Dimensions. — Alt. So mm.; lat. loS mm.; diam. 32 mm.; length of hinge-line, 45 mm. 



Found ill the Pliocene at Santa Barbara, California (Coniacl, Gabb, Yates 

 Cooper, Arnold). 



The description, measurements, and figures are of Gabb's type specimen of 

 Janira hella (No. 960, Cullection Academy Natural Sciences, Philadelphia), which 

 was kindly loaned to the writer by Professor H. A. Pilsbry, Curator of Mollusca. 



r. siearnsii and P. diegensia are distinguishable from P. hellus by the more 

 numerous (25 or 28 in the first, 20 or 22 in the second), narrower, sharper defined, 

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

 nently and evenly laraelhited ribs of the left valve. P. hemphilti is distinguishable 

 from P. helium by its smaller size; by having on the right valve more numerous (15 

 or 16 in the former), round-topped, narrower, nearly perpendicular-sided, radiating 

 ril«, which retain their prominence for their entire length, and by the less convexity 

 of the disk, more numerous, narrower and more elevated radiating ribs of the left 

 valve. P. hemphUli has the same depression below the apex in the left valve, but 

 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. hellns. P. deutalus is dis- 

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

 concavity of left valve, greater number of ribs, and by the auxiliary ribs in the 

 left valve. 



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

 tion and figure and Gabb's figure and type specimen, the writer has no hesitancy in 

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

 that Conrad's species and that of Gabb are different. This idea was probably caused 

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

 the specimens of P. heUaa 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. heUua ii\\t\ P. heinphi/ll show the dilYerences enumerated in a previous paragraph 

 to be constant foi- the adults. The young of both species up to an aUitude of twenty 

 millimeters are nearly identical in appearance. 



23. Pecten (Pecten) dentatus Hoicerhy. 



Plate XII, Figs. 1 and \a. 



Pecten dentatus Shy., Thes. Conch., Vol. I,]). 39, PI. XV, figs. 105, 106. D.\li., Trans. Wagner 



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

 Vola denlata Shy., H. &. A. Auams, Gen. Rec. Moll. 

 /anira denlata Sby. (=/ excavata, Val., fide Carpenter, Brit. Assn. Rept., 1863, p. 654). Gauh, 



Pal. Caj., Vol. II, 1869, p. 104. Cooper, 7th .\nn. Rept. Cal. St. Min., 1888, p. 244 {^P. 



sicarusii Dall, in part). 



