CLARK: THE CIDAKIDAE. 181 



Abactinal system not very sharply defined, rather 

 irregular in outline, witli reentering angles, 

 between ocular and genital plates ; oculars 

 with more or less concave outer margin or 

 deeply notched by ambulacra. Large glob- 

 iferous pedicellariae never as in Cidaris. 

 Abactinal system thick and solid, more or less 

 elevated ; genital and ocular plates with 

 more or less convex surfaces, thickly and 

 uniformly covered with tubercles of approxi- 

 mately equal size ; ambulacral secondaries 

 usually larger than those on genital, ocular, 

 and uppermost coronal plates and often 

 conspicuously contrasted with them. Cor- 

 onal plates few, 4-7, rarely 8 or 9 ; upper- 

 most 1 or 2 or even 3 without primary 

 spines. Primaries never smooth, but pro- 

 vided with longitudinal rows of granules, or 

 with ridges, 1 or more of which may be ele- 

 vated to form conspicuous, though delicate, 

 buttress-like "wings" along basal half of 

 spine; if these buttress-like "wings" are 

 not present, terminal portion of spine often 

 more or less fluted and flaring. Globiferous 

 pedicellariae, both large and small, com- 

 monly lack conspicuous end-tooth . . Stereocidaris 

 Abactinal system flat, usually not uniformly 

 covered with tubercles, some of which are 

 also larger than others ; ambulacral second- 

 aries not noticeably contrasted with others 

 abactinally. Coronal plates 6-8, rarely 9, 

 all (except usually uppermost 1, or rarely 

 2) with primary spines. Primaries some- 

 times perfectly smooth, never with " wings," 

 and seldom with flaring tip. Globiferous 

 pedicellariae, both large and small, com- 

 monly with conspicuous end-tooth. 

 Median ambulacral area .55 of ambulacrum 

 in width ; primaries shining as though 

 polished, white more or less shaded with 



greenish or pink, or both Calocidaris 



Median ambulacral area less than .50 of am- 

 bulacrum ; primaries never shining as 

 though polished Dorocidaris 



The above key gives little clue to the relationships of the genera with 

 each other, and a natural arrangement must necessarily be largely a 

 matter of speculation. There can be little question that Cidaris is 



