96 TERTIARY AND QUATERNARY PECTENS OF CALIFORNIA. 



of right valve ornaiiuMited by 1-i or 15 piominent. nearly Hat-topped, .s(iuare, radia- 

 ting ribs, some of them with one or two longitudinal obsolete lines; the ribs bec-ome 

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

 the adult; surface of right disk ornamented with close, tine, squamose, concentric 

 wrinkles; ears subequal. arched, covered with crowded, elevated lamella^; byssal 

 notch small. 



Dimensumx. — Alt. 8<i mm.; lat. los nun.; diam. 3'2 mm.; length of hinge 

 line, 45 mm. 



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

 Janira helhi (No. 960. Collection Academy Natural Sciences. Philadelphia), which 

 was kindly lent to the writer by Prof. H. A. Pilslny, curator of ilollusca. 



/'. ateanwi? and /'. diryi'iisis are distinguishable from 1'. h/Jlux by the more 

 numerous (25 or 2S in the tirst, 20 or 22 in the second), narrower, sharper detined, 

 perpendicular-sided, radiating ribs on the right valve, and b}' the evenly rounded, 

 prominently and evenly lamellated ribs of the left valve. P. h^mjiyhW). is distin- 

 guishalile from P. hellns 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 ribs, which retain their prominence for their entire length; by 

 the less convexity of the disk; and by the more numerous, narrower, and more 

 elevated radiating ribs of the left valve. /''. hernpliiUl has the same depression 

 below the apex in the left \alve, l)ut the lesser convexity of the rest of the 

 disk lessens the prominence of the depression, which is so marked in most speci- 

 mens of /'. heUuK. P. L'.vcai'dtnx is distinguishable from /'. helhis by its smaller 

 size, greater convexity of right valve, greater concavity of left valve, greater 

 number of ribs, and by the auxiliarj' 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. Dall is of the 

 opinion that Conrad's species and that of (xabb are difierent. This idea was 

 probably caused by the exaggeration of the bicarination of the ribs in Conrad's 

 figure. Several of the specimens of /\ hcllux examined by the writer show this 

 l)icarination to a greater or less degree, although as a rule the ril)s are nearly 

 smooth-topped. A large series of P. helhis and P. he)nphiUi show the difl'erences 

 enumerated in a previous paragraph to be constant for the adults. The .voung 

 of l)oth species up to an altitude of 20 millimeters are nearly identical in 

 appearance. 



P. hellus, so far as known, is confined to the Pliocen(\ It occurs abundantly in 

 the Pliocene formation on Packards Hill, Santa Barbara, and in other localities 

 in the vicinity. The type was obtained near Santa Barbara. 



RANGE. 



Pliocene. Santa Barbara (Conrad; Gabb; Yates; Cooper; Arnold); Top of hill. 

 Long Tom mine, Kern County (Watts); Eureka Canyon, west side, near 

 mouth, Ventura County (Eldridge); Kincon asphalt mine, Santa Barbaia County 

 (Watts); Log. 33, east side of Pirn Creek, near railroad bridge, Ventura County 

 (Watts); ? Temescal Canyon, north of Santa Monica, Los Angeles County 

 (Rivers). 



