PLIOCENE PECTENS. 125 



eated, and sculptured hy ol)soI<'te radials and tino iiicT-emental lines. I>eft valve 

 similar to right, except that the ribs are narrower and the interspaces corre- 

 spondingly l)roader. 



l)hiir)ix/oiix. — Alt. 30 mm.: long. '^\ nmi.: hinge lini^ 17 nini.: diameter 12 

 mm.: umhonal angle 95-. 



This species is characterized by the luunber of its riV)s and the concentric 

 imbricating bands of interrupted growth. It is distinguishable from /'. cerr(ixf'iiffi>< 

 by its fewer ribs, less prominent lirula- on the sides of the ribs, and its 

 prominent concentric bands of interrupted growth; distinguishable from P. 

 ch'cithtrix and varieties by its less \cntricosity, fewer I'ibs, and its characteristic 

 concentric sculpture. Specimens of tiiis species were noticed in a box in the 

 National Museum with /*. xiihfcDfrironiix \= P. cerri»<(')ix/x\ from Pacific Beach, San 

 Diego. 



P. rooperi is so far known only from the Pliocene of Pacific Beach, near 



San Diego. Type and cotype now in the collection of Delos Arnold. Named in 



honor of the late Dr. .F. G. Cooper, one of the pioneer paleontologists of the 



Pacific Coast. 



rax(;e. 



I'liocene. Pacific Beach, San Diego (Hemphill, Arnold). 



Pectkx (Plauioctenium) circuearis Sowerby. 



PI. Xr.II, fifff-. :!, 4, .=S, and ti; PI. XI. IV, figs, (i, ^ii, ()'<, and 7. 



ls:i.T. I'eclfti tuiiiidtt.'i Sowerl)y, Pmc. Zool. Soi'. Lnnd., 18.'i5. p. 109; ndt I'lctin Inmuliis Tnrtnn, 1S'_'2, 



nui- Zeiten, IS.Sll 

 ISoo. Fecten cirriUaris iiowev])y, Proc. Zuol. Soc. Lond., 18:i5, j). 110. 

 1,H43. Pecten ventricosus Sowerby, Tfie.-^. f'onch., vol. 1, pt. 1, Pecten, p. .51, pi. I'J, (igs. IS, U), ;ind L'ti; 



-j- P. tumiduK Sowerby, 1835. 

 184:-;. I'ecten clrcularis Sowerby, Thes. Conch., \n\. 1, pt. 1, Pecten, p. h\. No. 18, pi. 12, (ig. 23. 

 18-52. Pi'cten ren(ncos».s- Sowerby, Reeve, Coiu-h. Icon., v<il. 8, Pecten, sp. 31, Nov., 18.52, pi. 7, lias. 



31 ffl and }>. 

 18.53. Pecten circular^ Sowerliy, Keeve, Conch. Icon., vol. s, .lune, 1853, Pecten, f^p. 137, pi. 31, fig, i;;7. 

 1869. Pecten neyitrUvfuis Sowerby, (iabb. Pal. Cal., vol. 2, p. 104 { — P. var. .■njitisiilcatiif: Carpenter, in 



part). 

 1888. Pecten rentricosus Sowerby, Ki'ister and Kobel), C'onch.-Cab., vol, 17, lit. 2. sp, (i4, p. 100. taf. 28, 



figs. 1-3. 

 1888. Pecten (Penti pecten) circidnrh Sowerliy, Kiister and Kobelt, Conch. -( 'ab., vol. 17, [it. 2, sp. 151, 



p. 188, taf. 51, figs. 0-8. * 

 1888. Pecten )«n;rico.vH.f Sowerby, Cooper, Seventh Ann. Kept. Cal. St. Min., 1888, p. 258. 

 1898. Pecten (Plagiocte/nmm) ventricomix Sowerby, Dall, Trans. Wagner Free Inst. Sci., vol. 3, pt. 4, 



1898, p. 710. l+P. inca Orb., 1S47; ? = P. pornatia Val., Voy. Venus, 18;i5, pi, 19, fig. 3 (fi<le 



Dall, 1898.)] 

 1898. Pecten (Peclen) cumpactus Dall, Trans. Wagner Free Inst. Sci,, vol. 3, pt, 4. 1S98, p. 707, pi. 'M, 



fig. 5 (right valve of a young specimen). 



1903. Pecten { Plagioctrninm) centricosas Sowerby, Arnold, Mem. Cal. Acad. Sci., vol. 3, 190:!, p. 114, 



pi. 11, figs. 3, '4(1, 6, and 6o ( = P. var. nquisnlcatus Carpenter, 1863, in part). 



1904. Pecten venlricomi.s Sowerl)y, Keep, West Am. Shells, pp. 41—12. 



Description. — Shell a\eraging ai>out -IH uullimeters in altitude, about as long 

 as high, subequivalve, very convex, ineciuilateral, the posterior portion of the disk 



