46 Vniversity of California Publications in Geology [Vol. 12 



although derived from the scutellas, the time at which the division 

 took place is uncertain. 



Dendrastcr gibbsii has a comparatively long range, extending from 

 the Lower Etchegoin (Jacalitos) well toward the top of the Upper 

 Etchegoin. During this time it varies only slightly in the size of the 

 individual, probably due to difference in environment. In the Peeten 

 eoalingensis zone of the Upper Etchegoin the echinoids of the D. gibbsii 

 type seem to become slightly retrogressive in character. This is ex- 

 emplified by D. Jtcspcris Kew, which has a less eccentric apical system 

 than the earlier type, but, on the other hand, has become somewhat 

 thicker. The change in the eccentricity of the apical system here seems 

 to parallel the same sequence which occurs in the large group in which 

 D. cxcentncus is the last member. 



Among the first of the dendrasters is D. jacalitosensis Kew, which 

 is characterized by a relatively thin test, slightly eccentric apical 

 system, petals with broad interporiferous and narrow poriferous areas, 

 and marginal notches in the posterior ambulacra. It occurs in the 

 Lower Etchegoin (Jacalitos) formation in the lowest beds in which 

 fossils are found. The apical system, which is less eccentric than in 

 D. gibbsii, together with its stratigraphic occurrence, suggests that it 

 may be a more primitive form than the latter and more closely related 

 to the scutellas. The character of the petals do not necessarily indicate 

 this since in the California scutellas the poriferous areas are compar- 

 atively broad. 



The D. coalingacnsis group is represented by three species, the other 

 two being B. arnoldi Twitchell and D. perrini (Weaver). These 

 forms are very similar, the differences which distinguish them being 

 mainly in the thickness of the test and the arrangement of petals. 

 They are probably derived from the earlier dendrasters, and show a 

 distinct evolutionary series from D. coalingaensis to D. perrini, which 

 is somewhat more highly developed tlian the others. 



Toward the top of the Etchegoin formation, a different type of 

 dendraster appears, which is characterized by the thinness of the 

 test. The earliest foi'm possessing this character is D. gibbsii var. 

 humilis Kew, which gives rise to D. ashleyi (Arnold), a form which 

 represents the most highly specialized species in the development of 

 this genus ; it is characterized by an extremely eccentric apical system 

 and a broad odd anterior petal. This species was fairly widespread, 

 being present both in the Coalinga district and in the Santa Maria 

 oil district of Santa Barbara County. Near the latter district, in the 



