1920] 



Kew: Cretaceotis and Cenozoic Echinoidea 



45 



Simultaneously with the development of A. whitncyi in the north 

 and in the Salinas Valley area there was a form comparable to it which 

 had a much more comjjlex evolution in the southern San Joaquin 

 Valley area. This group is characterized by the species A. coalinga- 

 ensis Kew, which, though closely resembling A. whitneyi, differs in 

 being ornamented with larger tubercles, a feature also possessed by the 

 other members. The species arising from A. coalingaensis are A. 

 scutelliformis Kew, A. coalingaensis var. grmidis Kew, and A. cali- 

 fornicus Kew. From A. calif urnicus was probablj' developed A. 

 jacaUtosensis Arnold, a species living somewhat later in Lower Etche- 

 goin (Jacalitos) time. The migrooved interanibulacral areas and 

 more or less bell-shaped appearance of the last species ally it with 

 this group. In the Cuyama River district of Santa Barbara County 

 a species quite close to A. whitneyi, but possessing wider petals, is 

 present. Tliis form, A. cuyamanus Kew, probably represents con- 

 temporaneous evolution in a different geographical province. 



In summarizing the data on this genus, there is found to be a 

 main stalk descended from the genus Scutella by transitional forms, 

 which gradually evolve into more complex species XTutil the most highly 

 developed Asirodapsis is reached, after which the genus suddenly 

 becomes extinct. From this main line of descent at least four groups 

 have branched otf, the most important being the A. whitneyi and 

 A. arnoldi groups. Each of these four groups gave rise to evolu- 

 tionary series, and probably represent development along similar lines 

 in separate geographic provinces. 



DENDEASTEE SEEIES 



This genus was evidently derived from the scutellas, but through 

 what transitional forms it is at present not known. The first Denelras- 

 ter to appear was D. gihbsii (Remond), which is found earliest in the 

 strata of the Lower Etchegoin (Jacalitos) formation of the Pliocene. 

 The sudden appearance of a form with a markedly eccentric apical 

 system, a well advanced character, may be explained on the assumption 

 that it may have been an immigrant from another region where its 

 earlier evolutionary stages took place. The series (see fig. 5) may be 

 divided into two main groixps, that characterized by D. gibbsi and that 

 by 1). excentrkus (Eschsclioltz) . Only one small subsidiary group is 

 thrown off, that distinguished by B. coalingaensis Twitchell. Calaster, 

 the subgenus of Dendraster, has an entirely separate evolution ; 



