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University of California Publicatio'ns in Geology [Vol. 12 



furrows. The first is to be considered more especially in dealing with 

 the genus Dendraster. In the general evolution of the echinoderms as 

 a whole, the central apical system is more primitive than the eccentric 

 one, the latter occurring only in the Exocycloida. However, some of 

 the later forms show a less eccentric apical system than the earlier 

 types, tending to revert to a less specialized form. The supramarginal 

 position of the periproct in the exocyclic echinoids is a primitive char- 

 acter, since they have been derived from the Centrechinoidea, a regular 

 type, in which the periproct is on the abactinal surface. However, the 

 study of the Pacific Coast Scutellidae seems to indicate that the supra- 

 marginal position is also a retrogressive character in forms of a later 

 evolutionary stage. This is shown in the evolution of the west coast 

 scutellas, in which the later species, such as »S'. gabbi (Remond), ac- 

 quire the supramarginal periproct. This fact is also substantiated by 

 the same feature in the subgenus Calaster, which occurs in a compara- 

 tively late horizon where the more advanced characters should be 

 present. The size and shape of the test can be regarded as a function 

 of the environment. Since the Clypeastroids are forms which live 

 on the sandy bottom of the littoral zone, the roughness of the water at 

 any particular place may result in a thicker test to withstand the wave 

 action. A highly elevated abactinal surface may be produced so that 

 the petaliform portion of the test, in which the tentacles are specialized 

 as breathing organs, may be raised above the level of the sand which 

 buries the remainder of the shell. The size of the test depends to a 

 great extent on the food supply and temperature conditions, colder 

 water apparently dwarfing the individuals, as in Dendraster {Calaster) 

 oregonensis (Clark). It is mainly in the species of Astrodapsis that 

 the raised petals and interambulacral depressions are present, and 

 these are comparable to an advanced degree of specialization such as 

 occurs in many other groups of animals before their extinction. The 

 geologic succession also bears out this, for in the early forms, such 

 as Astrodapsis breweriamis (Remond), these characters are just 

 making their appearance and finally culminate, during the later hori- 

 zons, in the Astrodapsis arnoldi type. The ramification of the am- 

 bulacral furrows becomes more complex in the later forms, especially 

 in those of the genus Dendraster. An increase in depth of the furrows 

 is also a character which has been taken on during the later develop- 

 ment of the Scutellidae. 



