68 TERTIARY AND (JUATERNARY PECTENS OF CALIFORNIA. 



several constriftionb or uiidukitions of interrupted jfrowth; margins somewhat 

 coarsely serrate; 1)ase regularly rounded; sides straight, and forming an acute 

 angle at the umbo. Right valve with about 8 to 1<(, unequal, rather prominent, 

 flat-topped ribs, each of which is channeled by one or more deep, narrow, concave 

 sulcations which divide tl>e primary rib from uml)o to margin into two or more 

 generally unequal l)ut evenly con\'ex riblets; interspaces .somewhat narrower than 

 the ril)s, and containing from 1 to ;i pi'ominent rounded riblets; the riblets become 

 more numerous and the major ribs less prominent anteriorly and posteriorly; the 

 minor sculpture consists of scjuamose lines of growtli, which are more prominent 

 on the ribs, and Ijeautiful microscopic checkered tessellation which is best developed 

 in the major and minor interspaces; hinge line from two-fifths to nearly one-lialf 

 length of disk; anterior ear prominent and sculptured bj' 5 or G elevated radi- 

 ating ridges and imbricating concentric lamella^; byssal notch moderately deep; 

 posterior ear rectangularly truncated, a little less than one-half length of anterior, 

 and similarly but less prominently sculptured. Left valve with unecpial, rather 

 narrow, convex ribs, the alternate ones being more prominent than those inter- 

 mediate; extremities of th(> ribs sometimes ornamented by longitudinal riblets; 

 inter.spaces of about equal widtli to the ribs, and ornamented b}' one or three 

 alternately prominent intcrcalaries; surface sculptured by iuil)ricating incremental 

 lines; the constrictions, when present, are more pronounced in this valve; ears 

 sculptured similarly to those of the right valve. Hinge with a pair of prominent 

 cardinal crura adjacent to the umbonal pit in the right valve. 



Danenxions. — Alt. i'A nun.; long. 57 mm.; hinge line iJI mm.; diameter 21 mm. 



This species is allied to /••. jxtrmeleei and /'. m/tfsi but is di8tinguisha))le b}' 

 having a larger number of ribs, which are also narrower and more coarsely radially 

 sculptured. The Miocene forms of P. ludtcri have nu)re prominent and deeply 

 sulcated ribs on the right valve, are generally more convex and more prominentlj"^ 

 constricted, and have relatively longer posterior ears than the later forms. 



P. nutterK so far as known, ranges from the Mount Hamilton Range Miocene to 

 the lower Pliocene. It is found rather abundantly in a certain layer in the sea clitf, 

 one-half mile south of the mouth of San (xreg'orio Creek, San Mateo Count}', 

 associated with a characteristic' lower Pliocene (Purisima) fauna. The species is 

 named in honor of Mr. Edward Hoitt Nutter, who has rendered the writer much 

 valuable assistance in the i)reparation of this jiaper. The tj-pe is from San 

 Gregorio, San Mateo County, and is now in the collection of the department of 

 geology, Stanford University. 



RANGE. 



Pliocene (lower). Purisima to Pescadero, San Mateo County (J. P. Smith, W. R. 



Hamilton, Arnold); Kre3'enhagen's ranch, Fi'csno County (Watts). 

 Miocene (upper). Near Mount Hamilton, Santa CMara County (No. 5805, Cal. St. 



Min. Bur. Coll., collected by Mrs. A. E. Bush). 



