1920] 



Kew: Cretaceous and Cenozoic Echin-oidea 



37 



STATEMENT OF THE PHYLOGENY OF THE PACIFIC COAST 

 ECHINOID FAMILIES 



The Tertiary strata of California on account of their great thick- 

 ness and abundant fauna offer a remarkable sequence of fossil echi- 

 noids. The Clypeastrina is the only Cenozoic order which is repre- 

 sented by a sufficient number of species to permit of the working out 

 of an evolutionary series, the other orders, Cidaroidea, Centrechin- 

 oidea, and Spatangina, though present, being only sparsely represented, 

 and their occurrence is so scattered throughout the strata that a con- 

 tinuous succession is not obtained. From the clypeastroid material it 

 has been possible to work out not only the phylogeny of the family 

 Scutellidae itself (see fig. 2), but also that of three of its genera, 

 namely, Sciiiella, Astrodapsis, and Dendrastcr. 



The Scutellidae first appeared in the Tejon formation of the Upper 

 Eocene and the genus Scutella was its first representative. At first 

 the rise of this order was slow, and not until the Upper Miocene and 

 Lower Pliocene was its maximum develoi^ment reached. From this 

 time on it declined until at the present time only one indigenous 

 species, Scutella mirabilis (Agassiz), is living in the Pacific Ocean, 

 where it is confined to the Japanese and Siberian coasts and the Bering 

 Sea. During the history of the Scutellidae on the west coast three 

 genera, Scutella, Astrudapsis, and Dendraster, have played an im- 

 portant part. Besides these is Scutaster, a highly specialized form, 

 characterized by three lunules in the ambulacra of the trivium. It 

 had a very short existence in the upper part of the Oligocene, w^here 

 it is represented by a single species, Scutaster andersoni Pack. 

 Whether a form in such an advanced stage of development originated 

 in a relatively short time, as indicated by the existence of only one 

 species, or whether it had its earlier stages during the Eocene, is not 

 known, due to the fragmentary nature of the scutellid material in the 

 Eocene and Oligocene of California. 



Wliile the genus Scutella was at its maximum development in the 

 Lower and Middle Miocene the genus Astrodapsis was beginning to 

 make its appearance. In the evolution of this genus the species Astro- 

 dapsis hrewerianus (Remond) and A. brcweriantcs var. diabloensis 

 Kew are intermediate forms with characters pertaining to both genera. 

 Although in general these forms appear to be transitional in character, 

 nevertheless they possess some true astrodapsid features which dis- 

 tinctly separate them from the scutellas. A brief review of the dis- 



