48 TERTIARY AND QUATERNARY PECTENS OF CALIFORNIA. 



sedentaiT forms. The iiioveiiient, although quite rapid, is jerky, and is produced 

 by the expulsion of water through the cavity- of the auricles: the ventral margins' 

 of the valves being in advance as the aniiual moves. 



"In various geological horizons, as well as in the existing fauna, certain species 

 of Pecten assume a sessile habit, involving an irregular sutisequent growth of the 

 valves after attachment to other objects, as in Hinnites. These species have no nec- 

 essary genetic coimection with one another except what they gain from their rela- 

 tions to the Pectinidx as a group, and must be regarded as pureh' sporadic adjust- 

 ments of individual forms to a particular environment." 



"The influence of the environment is yery marked among the Pectens. As in 

 mammals and birds, the same species in the northern part of its range is larger than 

 in the south, unless it is a distinctively tropical species. But in color the rule is 

 reversed, the southern species being lighter and more l)rightly tinted than the north- 

 ern ones in the same species. The specimens which live in deep water and swim 

 actively are usually thinner shelled and smoother, while those which inhabit the 

 lagoons are heavier, have more conspicuous concentric sculpture and more solid 

 shells." 



The Pectens seem to form a natural genus which ma}- l)e separated for con- 

 venience into groups, but as might be expected when the fossil forms are taken 

 into consideration, these groups merge into one another by insensible gradations. 

 Whatever might be advisable were our knowledge of the Pvetinldnf confined to 

 the recent species, any paleontological division of them can not ignore the inter- 

 gradation which is so obvious between the difl'erent types of which the extremes 

 appear so uidike. 



The sul)divisions adopted here, although more or less widely separated in 

 the California fauna, are known elsewhere to be connected l)y intimate gradations. 

 For that reason the}' are assigned to a I'ank compatible with such relations. As 

 would be expected, the relative importance attached to the characteristics upon 

 which groups are founded is regulated more or less by the personal equation; 

 therefore, in order to avoid the complications often arising from a too close 

 discrimination, it has been deemed advisable in tills paper to make all of the 

 subdivisions which are woi'thy of a group name bear the grade of subgenus. 



The following gi'oups of Pecten are represented in the Tertiary and living- 

 fauna of California: 



iSubgenus Pecten s. s. Miiller. ♦ 



Type P. ina.ciinKs Linne. 



Right valve moderately inflated, left valve flatfish: sculpture of strong ribs 

 with radial striation. more or less roughened by simple concentric lamellation or 

 incremental sculpture; ears sube(iual. 



This group includes the following Pectens: auhurijl, helht-n, carriz(H'ni<!x, 

 coaling aensis, diegeTUiis, excavatnn^ hemphilll. la/epi, lecontei., meiTiami, mnctse- 

 cruzensis, stearnsii, and I'of/dexi. 



Subgenus Patinopecten Dall, 1898. 



Type P. cai(rinus Gould. 



Valves with .small ribs, flat on tlve right valve, and sometimes dichotomous; 



