72 TERTIARY AND Q0ATEENARY PECTENS OF CALIFORNIA. 



1894. Pecteu cslrellanus Conrad, Cooper, Bull. Cal. St. Min. Bureau, No. 4, pt. 5, 1894, p. 57 (in part). 

 1898. Pecten magnolia Dall, Trans. Wagner Free Inst. Sci., vol. 3, pt. 4, p. 702 (in part); not Pecten 



magnolia Conrad, Pac. R. R. Rept., vol. 7, 1857, p. 191, pi. 1, fig. 2. 

 1901. Pecten magnolia DsiW, Nautilus, vol. 14, 1901, p. 117 (in part); not P. magnolia Conrad, 1857. 

 1904. Pecten (Lyropecten) diabloensii Arnold, MS. (a distorted specimen of P. crasmrardo being the 



type). 



DescripfiiDi. — Adult, slu'll averaging' about 130 niilliineters in altitude, about as 

 long as high, inequivalve, convex, equilateral (except for ears), thick, and generally 

 prominently and abruptly constricted or laidulated one or more times near the 

 lunbo and sometimes at later stages of growth; sides nearly straight; base regularly 

 rounded; margins .smooth. Right valve with 1-t to 17 prominent, perpendicular- 

 sided, convex-topped ribs, which are separated by more or less deeplj' channeled, 

 flat-topped, convex-bottomed interspaces somewhat narrower than the ribs; whole 

 surface of ribs and interspaces sculptured by numerous radial strict? and tine, 

 minutely wavy, imbricating, concentric lines; hinge line about four-sevenths length 

 of disk; anterior ear somewhat produced, subarcuate laterally and sculptured by 

 about 5 prominent radial ridges and numeroits tine imbricating incremental lines; 

 posterior ear about three-fourths length of anterior, sculptured similarly, but 

 slightly less prominently, and rectangularly truncated. Left valve similar to right, 

 except that the ribs are narrower, and the interspaces correspondingly broader; 

 ears .similar to posterior one of right valve. Hinge with a deep resijial pit. on 

 each side of which are several prominent lateral crura or teeth. 



Diiiiensimis. — Alt. 140 mm.; long. 138 mm.; hinge line 85 mm.; diameter 50 

 nnn.; lunbonal angle 105^. 



The tops of the ribs sometimes overhang the interspaces and also sometimes 

 appear smooth. Great variation is shown in regard to the number, abruptness, and 

 prominence of the constrictions on the disk. One or two are generally present near 

 the umbo in most .specimens, and some show as many as six or seven at different 

 periods of growth. The specimens of this specii^s from Orange County are generally 

 freer from constrictions and the disks are usuall v more compressed than in the forms 

 from central California; otherwise the former are normal types. 



P. crmsicardo is characterized by its comparatively few ribs, which, with the 

 interspaces, are more or less prominently striated. It is distinguishable from P. 

 uxkh'i//\ by its relatively much greater length, fewer ribs, and tendency to be con- 

 .stricted at different stages of growth; distinguishable from /'. m'U/noI!.a, with which 

 it is often confused, by its more numerous and much narrower ribs, relatively shorter 

 hinge line, more prominently striate ears, and its tendenc)' to become constricted; 

 distinguishable from P. estrellanus, with which it is often associated, by its fewer 

 and rather tinely striate ribs and lack of a major intercalary riblet (the bottoms of 

 the interspaces of P. erassieurdo being striate rather than containing a riblet). Some 

 of the lateral ribs and interspaces, and the surface near the periphery of the disk in 

 adidts of P. estrellarms generall3^ show sculpture similar to that characteristic of 

 P. viussicardo. 



P. crassicardo is found abundantly in the San Pablo, or upper ]\liocene forma- 

 tion of central California, and also in beds of possibly lower Miocene age in .southern 

 California. 



