MIOCENE PECTENS. 89 



Dimeuxioiis. — Alt. 35 mm.; long'. 35 mm.; hinoe line 20 mm.; diameter *i mm. 



The mea.surements given above are from an average-sized specimen. One left 

 valve examined was 50 millimeters in altitude, but this size is exceptional for 

 specimens from the type locality (shore of San Pablo Bay). 



The prominence of the ribs seems to i)e the most variable factor in this 

 .species. As a rule the ribs are sharp and well detined until the shell attains an 

 altitude of 10 or 15 millimeters, when they broaden out and become low, those 

 near the sides almost losing their identity in some specimens. One specimen 

 examined retained the elevated and sharply detined ribs up to an altitude of over 

 25 millimeters, and very much resembled a young /■'. eircularis var. ^fju/.sidcadis. 



P. jxMoensis is so far known onlv iti the San Pablo, or upper Miocene 

 formation. At the type locality, on the shore of San Pablo Bay. Contra Costa 

 County, a bed several feet thick is made up almost entirely of valves of this 

 species. At this locality P. puMoenxix is associated with /*. cra-ssicardo and 

 P. iinderson! {i). 



RANGE. 



Miocene (u2)pcr). Shores of San Pablo Bay, Contra Costa County (Newberr\-; 

 Merriam; Arnold). 



PeCTEN (PsEUDAMUSIUM) LOMPICOENSrS n. sp. 



PL XXIII, lig. .5; PI. XXVII, fig. 8. 



Descrijytlon. — Adult shell averaging about 11 millimeters in altitude, subcir- 

 cular, inequi valve, thin, compres.sed; margins smooth. Right valve ornamented 

 externalh' liy luimerous sharp,, elevated, equal, equidistant, imbricating, concentric 

 lamella;; hinge line equal to one-half length of disk; ears unequal; anterior nearly 

 twice as long as the posterior, arcuately truncated and sculptured by regular, 

 shar]:), imbricating, concentric lamelhe and *; or T faint radials; posterior ear short, 

 somewhat ol>liquel3' truncated and sculptured bj' concentric lamelhe; byssal notch 

 only faintly indicated. Left valve sculptured by concentric lamellae similar to those 

 of the right, and, in addition, having numerous tine radiating strise, which usually 

 become obsolete near the periphery in the adult shell; ears .similar to those of the 

 right valve, except that in the specimens examined no radials were noted on the 

 anterior one. Hinge similar to that of P. randoJphi.. 



Dimensions {of slightly distorted specimen). — Alt. 11.5 mm.; long. 11 mm.; 

 hinge line 5.5 mm. 



This beautiful little Pecten is allied to P. peck/mm I, but is distinguishalile 

 from the latter b\- its much more prominent and regular concentric lamelhe, and 

 b}^ its well-defined ears, the anterior one of which on the right valve is relatively 

 much shorter and broader than the analogous one in P. pvclxhaml. The bvssal 

 notch in /■■. lomplcoensls is also only faintly indicated, while in 1'. peckhaml the 

 notch is profoundly developed. 



/-*. lomplcoensls is known only from the Miocene (exact horizon doubtful) 

 shales at the head of Lompico Creek, Santa Cruz County, where it is associated 

 with a Phacoldes resembling acutilineatus Conrad. 



RANGE. 



Miocene. Head of Lompico Creek. Santa Cruz County (Newsom. Arnold). 



