CLARK: THE CIDARIDAE. 171 



In 1907 A. Agassiz and Clark published descriptions and numerous 

 figures of nine new species of Cidaridae and instituted two new genera, 

 Auomocidaris and Aporocidaris. They also furnished much additional 

 information concerning Stephanocidaris, Centrocidaris, and Acanthoci- 

 daris and in regard to diversity of form in the pedicellariae of the 

 group. 



Fundamental Principles for a Natural Classification. 



Before attempting to set forth a revised classification of the Cidaridae, 

 if it is hoped to have it stable and generally acceptable, one ought to 

 make plain the principles on which it is based. These principles must 

 take into account not only the characters afforded by the specimens 

 themselves and the proper estimation of the relative value of these, but 

 also the selection of names for the genera and species held to be valid. 

 Fortunately there is coming to be more and more general agreement 

 among zoologists as to the principles which should govern in the selec- 

 tion of names, and the very general acceptance of the International Code 

 of Nomenclature, at least in its essentials, indicates clearly the approach 

 of the time when nomenclature will be fixed. In the following pages 

 adherence has been given to the rules of the International Code, but 

 whenever there has been room for difference of opinion as to the appli- 

 cation of those rules, that course has been followed which would cause 

 the least possible change from currently accepted names. Consequently 

 there are few changes from the names established or indorsed by A. 

 Agassiz in the "Revision of the Echini" and almost universally used in 

 the last quarter of the nineteenth century. Unfortunately there is no 

 code by which can be determined the relative importance of the various 

 characters which distinguish the different species and genera of Echini. 

 Here each writer is thrown upon his own resources, and his proposed 

 classification will stand or fall according to the judgment he displays 

 in selecting stable and significant characters. The fundamental diffi- 

 culty with the classification of Mortensen is that it is based almost 

 wholly upon the characters of the pedicellariae alone, and the history of 

 zoology shows again and again that a classification based on a single char- 

 acter, however suggestive it may be, is never reliable. The characters 

 afforded by the pedicellariae are important, but those organs are, like all 

 calcareous formations among echinoderms, liable to great diversify. It is 

 of no special importance in this connection whether the pedicellariae are 

 modified spines or not, the only point being whether, like the spines, 



