1920] Kew: Cretaceous and Cenozoic Echinaidea 35 



nevertheless this type resembles some of the more tropical forms, such 

 as Clypeaster ; and from this similarity it may be inferred that warm 

 temperate waters were present during the rapid development of this 

 genus. A temperate climate, though probably somewhat cooler, con- 

 tinued into the Pliocene, at which time the genus Dendraster and 

 subgenus Calaster reached their maximum development. In this con- 

 nection it may also be of interest to note that Calaster interlineatns 

 and C. oregoneiisis never have been found south of Monterey Bay, 

 though they live northward along the coast of Oregon, indicating 

 that these forms were restricted to cooler water than such species as 

 Dendraster gibhsii (Remond) and D. ashleyi (Arnold), which do not 

 occur north of Monterey and are never found associated with the for- 

 mer species. During the upper Pliocene and Pleistocene D. excentricus 

 (Eschscholtz) and Strongylocentrotus afford no definite information 

 as to climatic conditions other than that it was approximately the 

 same as the present. An exception to this is the occurrence of Mellita 

 longifissa Michelin at Newport Beach, near Los Angeles, in beds of 

 late San Pedro age. The presence of this subtropical form is direct 

 evidence that a warm water condition existed at this time as far north 

 as San Pedro. 



From the above facts it is evident that no very definite conclusions 

 can be reached concerning the geographic distribution of echinoids by 

 temperature control through comparison with the Recent forms, except 

 in a few cases. Still, if considered with other groups of life, the 

 echinoids may be of considerable importance in this connection as a 

 link in the chain of evidence bearing on the problem of the climatic 

 history of the Cretaceous and Tertiary epochs of the Pacific Coast. 



PHYLOGENETIC SERIES 



PEINCIPAL CRITERIA USED IN CONSTRUCTION OF PHYLOGENETIC 



SERIES 



Some of the more important criteria to be considered in the dis- 

 cussion of the evolution of the Pacific Coast echini are: (1) the eccen- 

 tricity of the apical system; (2) the position of the periproct ; (3) the 

 size and shape of the test at maturity; (4) the amount of elevation 

 of the abactinal portions of the ambulacra and the depression in the 

 interambulacra ; and (5) the degree of ramification of the ambulacral 



