180 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Globiferous pedicellariae present, but often only small ones. 



Abactinal system very large (.60-70 h. d.); ambulacral 



plates few, generally less than 30; poriferous zones 



not at all sunken; secondary and miliary spines 



alike, cylindrical and more or less club-shaped; no 



tridentate pedicellariae present Aporocidaris 



Abactinal system less than .60 h. d. ; ambulacral plates 

 more than 40 (except, of course, in young indi- 

 viduals). 

 Abactinal surface conspicuously bare, with no primary 

 spines or well-developed tubercles or areolae much 

 above ambitus; no tridentate pedicellariae pres- 

 ent Anomocidaris 



Abactinal surface not so conspicuously bare ; at least two 

 primary spines well above ambitus in each interradius. 

 Areolae little or not at all sunken ; actinostome gen- 

 erally larger than abactinal system, which is usu- 

 ally .40-.45 h. d. ; median ambulacral area with 

 only a single marginal series of tubercles, though 

 there are usually other smaller, scattered tubercles 

 between, and these may form 1-5 vertical series. 

 Primaries .65-1.60 h. d. but commonly about equal 

 to h. d., rather stout, usually blunt; secondaries 



broad, flat, and truncate Cidaris 



Areolae more or less deeply sunken ; actinostome usu- 

 ally smaller than abactinal system; median am- 

 bulacral area usually with a double marginal series 

 of tubercles, inner much smaller. Primaries 1-3 

 h. d.; secondaries diverse. 

 Small (25-40 mm. h. d.) ; abactinal system with few, 

 generally less than 200, tubercles ; secondaries, 

 especially ambulacral, rounded, thickened, and 

 more or less club-shaped ; no tridentate pedicel- 

 lariae ; large globiferous pedicellariae with no 

 end-tooth on the valves. Subantarctic, north to 



about 35° S Austrocidaris 



Larger (30-70 mm ) ; abactinal system with more 

 numerous tubercles ; secondaries flat and thin, 

 and usually narrow. Tridentate pedicellariae 

 usually present and large globiferous, often with 

 an end-tooth on the valves. Northern hemi- 

 sphere, seldom south of the equator. 

 Abactinal system sharply defined, more or less dis- 

 tinctly circular or pentagonal in outline; ocular 

 plates with outer margin convex or straight, 

 little notched by ambulacra. Some or all of 

 large globiferous pedicellariae, if not like small 

 ones, have curved valves, large terminal open- 

 ing, and no end-tooth, as in Cidaris . . Tretocidaris 



