82 TERTIARY AND QUATERNARY PECTENS OF CALIEORNIA. 



The range of this species is not definitely known. It is found in the 

 Miocene, probably lower, of the Ojai Valley. Ventura County. The type, which 

 was collected at the above locality by Dr. Stephen Bowers, is now in the 

 collection of Delos Arnold. The species is named in honor of Dr. T. Wayland 

 Vaughan, of the United States Geological Survey. 



RAXGE. 



Miocene (lower). Ojai Valley, Ventura County (Bowers). 



Pecten (Plagioctenium) andkrsoni n. .sj). 



PI. XXVI, figs. 5, .5u, 6, 7, 8, and Sn. 



1S94. Pecten discus Cooper (not Conrad), Bull. Cal. St. Min. Bureau, No. 4, pt. rt, 1894, p. .57, pi. 5, 

 figs. 5.5 and .56 (in part). 



Description. — Shell averaging about 85 niillinieters in altitude, generally some- 

 what longer than high, both valves convex, moi'e or less inequivalve and inequi- 

 lateral; margins smooth; base regularly rounded; sides concave above. Right 

 valve with 1-i to IS prominent rounded ribs, separated by flat-bottomed inter- 

 spaces, which are generally of about equal width to, but sometimes narrower than, 

 the ribs; surface scidptured by numerous, sharp, imbricating, incremental lines, 

 and sometimes, at irregular intervals, by more or less prominent lines of inter- 

 rupted growth; hinge line more than one-half length of disk; ears subequal; 

 anterior ear with 3 or \ sharp radials and numerous fine concentric lines; 

 byssal notch quite pronounced; posterior ear sculptured similarly to anterior, and 

 truncated nearly rectangularly. Left valve more convex than the right, sculptured 

 similarly and with similar ears, except that the notch in the anterior is not .so 

 pronounced. 



Dimensions. — Alt. 37 mm.; long. 44 nun.; hinge line 23 mm.; diameter about 

 S mm.; umbonal angle 10.5'. 



A prominent characteristic of this species is that the left is decidedly the 

 more convex of the two valves. In this respect it is similar to P. latiauritus 

 and varieties. In fact, the young of P. andersoni is almost indistinguishable, in 

 •some instances, from small specimens of P. latiauritus var. numotiineris. The 

 young of P. andersoni are eq^uilateral, are relatively more convex, and have more 

 prominent ribs than the adult forms. 



Those specimens of P. andersoni found in the earlier formations appear to 

 be somewhat smaller, and have on an average fewer ribs (14 to 17) than tho.se in 

 the later Miocene horizons, which have from 15 to 18. Exceptions to this rule, 

 however, are not rare, one specimen, for example, from the same locality as the 

 type (which has 16 ribs on the left valve) having 18 ribs. 



It would not be surprising to the writer to find, after an examination of a 

 large series of /*. andersoni and allied forms from the upper Miocene, that it is 

 more or less closely related to P. discus. Large specimens of P. andersoni very 

 closely resemble some specimens of P. discus but ditt'er from the latter in the 

 fewer number (14 to 18) of their ribs, P. discus having 18 to 21. 



