78 TERTIARY AND QUATERNARY PECTENS OF CALIFORNIA. 



Description. — Shell averaging about 150 millimeters in altitrde, slightly lounger 

 than high, subequivalve, equilateral, rather thick, somewhat ventricose, and with 

 smooth margins; base regularly rounded; sides sloping above, and slightly concave. 

 Right valve slightly more convex than left, with about \) to U broad, flat-topped, 

 equal, equidistant ribs; interspaces slightly narrower than ribs and flat-l>ottomed; 

 whole surface of ribs and interspaces covered by imbricated radiating riljlets (about 

 10 on each rib) and tine, concentric incremental lines; hinge line about two-thirds 

 the length of the disk; anterior ear slightly longer than the posterior, with arcuate 

 end and tine concentric incremental sculpture, l)ut obsolete radiating ridges; posterior 

 ear with no radial sculpture whatever; bvssal notch relatively' small. Left valve 

 quite similar to right but with narrower, more rounded ribs; sculpture of radiating 

 riblcts and line wavy incremental lines; interspaces broader than in right valve; 

 ears almost identical to those of right. 



Dimensions. — Alt. 146 mm.; long. 150 mm.; hinge line 105 nun.; diameter 

 65 mm. 



The original description and figure of this species are poor, but there is no 

 question in the writer's mind as to which species Conrad applied this name. There 

 is no other large Pi-cten in the Miocene of the West Coast ha\'ing the small number 

 and characteristically large, striate ribs, and ears with so little radiate sculpture. 

 The figure of the type, poor as it is, shows that there was little or no radiate 

 sculpture on the ears of the specimen or specimens serving as the type for the 

 drawings. Furthermore, there is no other Pecten, except the species al)ove 

 described, which would bring out the following comment, which Conrad, in his 

 original description, applied to P. magnolia: 



"This species will compare in size with /'. Jcffersonius Say, and is so remark- 

 ably similar to it that it may prove to be the same species when more perfect speci- 

 mens are collected." 



The specimens which served as the tvp'es for Conrad's P. vtagnol/a are in the 

 U. S. National Museum (Nos. 13311 and 133ii5), and came from the Santa Inez 

 Mountains, Santa Barbara County. Although much broken, these specimens show 

 the ribbing and sculpture of the original shell almost perfectly. 



In many of the worn specimens of this species the radiating striis have been 

 lost, and the ribs appear smooth. Internal casts shovv smooth rounded ribs for 

 both valves. The distinguishing characteristics of this magnificent species, some 

 of which attain an altitude of over 210 millimeters, are its size, small number and 

 equality of ribs, equality of ears, and almost total lack of any radiating sculpture 

 on the ears. 



P. magnolia is somewhat closely allied to P. crass icardo, and is no doui>t its 

 precursor. It is distinguishable from the latter by its fewer (!» to 11, instead of 

 14 to 17), broader, more square-cut and more coarsely radialh' sculptured ribs, and 

 the obsolete radial sculpture on its ears. 



P. 'magnolia is one of the most characteristic of the lower Miocene, or Vaqueros 

 formation, fossils. In the region around Mindego Hill, San Mateo County, it is 

 found in the Vaqueros sandstone associated with the following fauna: Agasoma 

 harkerianuni, Agasoma, (qf.) gravida, Agasoma kernianuni. Area (cf.) mierodonta. 



