84 TERTIARY AND QrATEKNARY I'ECTKNS OF CALIFORNIA. 



larlv iduiulcd: sides noiirly struight. sloping at a iiKulciatcly low angle. Left valve 

 witli 17 lather narrow, prominent rounded ribs, each with a roiuided, thread-like 

 rihlet rnnning longitudinally along its top; at more or less regular intervals small 

 obsolete spines protrude from the riblet: interspaces wider than the ribs, with 

 sloping si(h's. and flat bottoms: whole surface of disk crossed bv very tine incremental 

 lines: hinge line longer than one-half length of disk: ears subequal. rectangularly 

 truncated, and sculptured by radiating ridges and incremental lines. 



J)iiii,'iis!onx. — Alt. 40 nun. ; long. 44 mm.: hinge lin(> :iS mm. : diameter 7 mm. 



This variety, which is characterized by the thread-like riblet along the top of 

 each primarv rib on the left valve, has been found only in beds of lower Miocene 

 age {TiifrifeUa i>i-(>;/iui(( zone) in the southern San Joaquin country. It is found 

 associat(Ml with the following characteristic fauna in the bhitls of the Kern River, 

 1 mile below the power-dev(>lopmeut station. Kern County: Conux n. sp.. Maetra 

 WGAV j>laiiHlata^ I'hucolilcs sp., Ttllina near id;v, Trochita Jilosa, Aydsonut l-ejmi- 

 anuiii, A(/asoiiia Ixtrkertaninn. Lidn taphrla, Leda (<{/'.)./""•"««. Vanct'llarut n. sp., 

 Plimrotoiiia {DoUchototna) n. sp.. Phnirvtoma {Leucosyrinu) near pedroana, Ohione 

 n. sp. near gnidla, DevtaUuin 2 n. sp., Nama {cf.) perpuKjnix, Ph'urotoma sp., 

 Neptimea n. sp., Cntcihuhiui n. sp., Turritdla ocoyana, ('(mcellarm {cf.) vefmta, 

 Phacoidfs (cf.) iiiiftiilli, Xiittca or Limatia sp., Terehra sp. (probably new), Trophon 

 (if.) jiiniihniKuiii, Drillia (nf.) iorosa, Solen. {af.) siearn/s. At Avenal Canyon, 

 Kei-n t'ounty, Mr. Eldridge found P. var. barJcen"anu.'< v.\t\\ J', mtdersoni, TiirriteUa 

 oroyana., Dodnia (large and Hat), Yenm sp.. and (Mrca sp. 



The type of P. var. harkerianus was found cast of the Barker ranch house, 

 near Kern River, Kern County, and is now in the collection of the California 

 Academy of Sciences. 



RAXGE. 



Miocene (lowe)-). Barker ranch, Kern County (Watts): Avenal Canyon and bluti's 

 of Kern Rivei- near power-development station. Kern County (Eldridge). 



PeCTEN (Pl.AOIOCTF.NU'Ml CKUKOSENSIS OAnH var. ( MENDENHALl.l n. Var. 



I'l. XXV. ligs. 2, •>(!, and ih. 



DeM-ripfiwi. — Sh(dl. when adult, averaging about 75 njillimeters in altitude. 

 Similar to /*. cerronKUxls in shape, convexity, and ril>bing. l)iit differing from the 

 latter in being much smaller when adult, having fewer ribs (about ly in the former, 

 while the latter has usually 21 or more), much less prominent incremental lines, 

 and a relatively longer hinge line. 



Dlnu'ii.s!(m>i {of a uiedliiin-!<ized. specimen). — Alt. 4;:>mm.; long. 44 mm.: hinge 

 line 28 mm.; diameter 17 mm. 



Although this form is closely allied to /'. ct/vv/.yfW •■<•/*■, it. nevertheless, should 

 possil)ly have specific rank, as the ditlerences between it and the typical P. ceiroxcit-ns 

 are constant, and it belongs to a different geologic horizon. Not enough is known 

 about its range and relations, however, to wairant a wider separation, at present, 

 than that of a variety. 



The type is from beds of probable Miocene age (the equivalent of the Carrizo 

 Creek beds) at Santa Rosalia. Lower California, directly west of and across the Gulf 



