48 



Vniversity of California Piiblications in Geology [Vol. 12 



It has a somewhat larger aiid heavier test, which may have been 

 acquired by inhabiting a region more exposed to the waves. The 

 narrow angle between the petals of the bivium, one of its specific 

 differences, cannot be accounted for at present through any evolution- 

 ary change. At the same time that D. diegoensis diegoensis was living 

 in the vicinity of San Diego a subspecies, D. diegoensis venturaensis 

 Kew, was present in the region of Ventura County. This form closely 

 resembles the former yet differs in being somewhat more specialized 

 in haviiig a markedly elevated abactinal surface within the radius of 

 the petals. Another difference is in the slightly raised petals, a char- 

 acter which was found to be an indication of advancement in the 

 dendraster series. 



The Recent Dendraster excentricus shows a slightly different ar- 

 rangement in the angulation of the petals from that in D. diegoensis, 

 but the main progressive character is probably the more extended 

 poriferous areas of the petals. 



The subgenus Dendraster (Calaster) is short-lived and includes 

 but three forms, which iuhabited only the cool waters of the Pacific 

 Coast. It was probably derived directly from the scutellas, as D. 

 (Calaster) orcgonensis (Clark) shows relationship to them in having 

 only a slightly eccentric apical system and in that the distinguishing 

 subgeneric character, the supramarginal periproct, appears to be a 

 further advancement of a similar feature which is found in the later 

 scutellid forms, such as 8. gahbvi (Remond). Although D. (Calaster) 

 oregonensis, together with D. (Calaster) oregonensis var. gibbosus 

 Kew and D. (Calaster) oregonensis var. major Kew, is stratigraphi- 

 eally lower than the other species, D. (Calaster) interline at u§, the 

 latter seems to be somewhat retrogressive in having a less eccentric 

 apical system. As has been noticed in tracing out the history of the 

 other dendrasters, the later forms seem to have less eccentric apical 

 systems, so this group may be similar in this respect. The much 

 greater size of the test probably indicates a more favorable environ- 

 ment. 



Summarizing the development of the genus Dendraster and the 

 subgenus Calaster, their history begins with relatively well advanced 

 forms which appear to be introduced suddenly as immigrant species, 

 then after reaching a high stage of specialization they decline some- 

 what, until finally only one species is left, D. excentricus, which is 

 probably a retrogressive form. 



