176 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The genus Dorocidaris is difficult to separate, on the one hand, from 

 Cidaris, and on the other from Stereocidaris, but is particularly close to 

 the latter, and it is almost impossible to draw a sharp line between 

 them. Moreover, it contains a rather heterogeneous lot of species. 

 One of these, D. micans Mortensen, seems to be quite unique, and I think 

 it may well be made the type of a new genus for which I would suggest 

 the name Calocidaris- The remaining species fall naturally into three 

 groups, distinguished from each other by their abactinal systems, spines, 

 pedicellariae, and distribution. I see no objection to recognizing these 

 groups as genera, and such a course has some obvious advantages. A 

 typical Dorocidaris such as papillata has the abactinal system irregularly 

 angular and often indistinctly defined, and the globiferous pedicellariae 

 have a conspicuous end-tooth on each valve. But other species have 

 the abactinal system circular or pentagonal and sharply defined, and 

 some of the globiferous pedicellariae are often more or less like those 

 of Cidaris. To this group D. hartletti A. Ag. belongs, and as Morten- 

 sen has made that species the type of a new genus, Tretocidaris, that 

 name must attach itself to this section of Dorocidaris, even though 

 few of the species have the remarkable pedicellariae which Mortensen 

 considers the distinguishing character of the genus. Finally, a group 

 of three small species, characterized by their thickened secondaries, 

 globiferous pedicellariae without end-tooth on the valves, sparsely 

 tubercled abactinal system, and antarctic or subantarctic distribution, 

 may be conveniently designated as Austrocidaris. The table on the 

 opposite page gives the genera adopted in the present paper, with their 

 authors, the year in which they were proposed, and the type-species of 

 each. The number of recent species in each, which seem to me valid, is 

 also indicated. 



The number of fossil specimens to which specific names have been 

 given is in the vicinity of 200 ; of these, Doderlein lists 135, but there 

 is reason to believe that many of these represent different ages or indi- 

 vidual forms of single species, and it is not unfair to assume that the 

 number of extinct species actually known to science does not exceed 

 the number of species now living. The following key will bring out the 

 obvious if not the most important characters by which the 21 genera 

 here recognized may be distinguished. It is hoped that such a key may 

 be of use to palaeontologists as well as to zoologists. The dimensions 

 are given in millimeters, and the horizontal diameter of the denuded test 

 (abbreviated for convenience to " h. d."), taken at the ambitus, is used 

 as the unit for determining the relative proportions of the various 



