CLARK: THE CIDARIDAE. 173 



bathymetrioal distribution. Even the suggestions of size, color, habitat, 

 and habits have not been ignored in the effort to learn the real interrela- 

 tionships of the species. At the suggestion of Mr. Agassiz, I have in- 

 cluded the genera of fossil Cidaridae, as well as the recent forms, in 

 order that the result may be as useful to palaeontologists as to zoologists, 

 and I have endeavored to give special consideration and due weight to 

 those characters upon which palaeontologists are obliged to rely I am 

 forced to the conclusion, however, that in most cases little value attaches 

 to the presence or absence of crenulation on the tubercles, to the straight- 

 ness or sinuosity of the ambulacra, or to the amount of confluence of the 

 areolae. While these features are frequently very obvious in fossils, ex- 

 perience with large series of specimens shows that they are very variable 

 in individuals of the same species, and the most striking differences may 

 be due to the age or condition of the specimen. Far be it from me to 

 claim that the genera which I have adopted are all of equal value or that 

 they ought to be adopted as herein denned by all future writers. The 

 genera Phyllacanthus and Stereocidaris are notably unsatisfactory, and it 

 is quite likely that they will be entirely rearranged in the light of further 

 knowledge. Perhaps the same is true of Goniocidaris. But it is hoped 

 that the classification and nomenclature set forth in the following pages 

 may be a real step towards the ideal which we seek 



The Genera, 



In attempting to apply che principles outlined above, it will be con- 

 venient to begin with those genera which are accepted by A. Agassiz, 

 Doderlein, Mortenseu, and Pomel, and virtually by Duncan also. These 

 genera are : — 



Cidaris Leske. Porocidaris Desor. 



Goniocidaris L. Agassiz et Desor. Phyllacanthus Brandt. 



Doderlein ( : 06) has reached the very disturbing conclusion that 

 papillata is the type of Cidaris, and that consequently Dorocidaris 

 A. Ag. is a synonym of Cidaris Leske. Acting on this belief, he has 

 introduced Lamarck's name Cidarites for Cidaris as commonly used, and 

 divides it into three subgenera, to one of which he applies the name 

 Dorocidaris A. Ag. In doing this, Doderlein overlooks the very im- 

 portant fact that Leske's Cidaris papillata is a composite group which 

 was first broken up by Lamarck. It includes at least three species, — 

 imperialis, which Brandt removed to Phyllacanthus; papillata, which 



