CLARK: THE CIDARIDAE. 179 



Ambulacral and interambulacral plates without such pits. 

 Abactinal system of numerous thin plates, with very large anal sys- 

 tem around which ocular and genital plates form a single narrow 

 ring; genitals, except madrepore, much wider than high, often 

 twice as wide ; oculars nearly as high ; collar of primaries spotted 

 with white; lowest actinal primaries with very wide collar and 

 a short thick cap of outer layer of spine, flattened, curved, and 

 Bomewhat serrate at tip, when fully developed . . . Stephanocidaris 

 Abactinal system not as above; collar of primaries not white- 

 spotted ; actinal primaries not provided with a distinct cap. 

 Median interambulacral area less than .30 of interambulacrum 



Phj/Uacanthus. 

 Median interambulacral area more than .30 of interambulacrum, 



densely covered with minute tubercles Chondrocidaris 



Pores nearer together, usually more or less oblique, often separated by an 

 elevation and never yoked together by a groove. 

 All primary tubercles large, smooth, and imperforate .... *Tylocidaris 

 Primary tubercles, at least at ambitus, perforate. 

 Ambulacra more than half as wide as interambulacra .... Centrocidan's 

 Ambulacra not half so wide as interambulacra, usually much less. 

 Coronal plates with areolae so small their diameter is less than one- 

 quarter horizontal length of plate and only about one-half verti- 

 cal height *Orthocidaris 



Coronal plates with areolae which occupy a large proportion of plate. 

 Ambulacra broad, .35-. 45 of interambulacra, with median area 

 correspondingly wide, sometimes sunken and more or less 

 bare; median space of interambulacra, especially along verti- 

 cal, and inner portion of horizontal sutures, sunken deeper than 

 areolae, especially at angles, and more or less bare ; in some 

 species, however, miliary tubercles cover so much of inner half 

 of each coronal plate that parts of vertical suture are concealed 

 and only short, bare, horizontal furrows are visible, and even 

 these may be only faintly indicated. Coronal plates numer- 

 ous in proportion to h. d., 6-11. Primaries always rough and 

 more or less thorny or prickly, often flaring at tip . . Goniocidaris 

 Ambulacra less than .35 of interambulacra, or, if more than that, 

 primaries not thorny. 

 Coronal plates numerous and narrow, 9-15, with areolae merg- 

 ing into each other throughout the whole series . . * Pohjcidaris 

 Coronal plates rarely more than 9, areolae at ambitus and abac- 

 finally never merged together. 

 Primary spines long, 2-3 h. d.,not at all thorny or prickly, broad 

 and somewhat depressed at base, tapering much but gradu- 

 ally , of ten slightly curved, and with a conspicuous light-col- 

 ored or spotted collar, one-fifth or more of the length Acanthocidaris 

 Primary spines very diverse, but never as above. 

 Only tridentate or, more rarely, bidentate, pedicellariae 

 present, but these abundant and often very large 

 (2-6 mm. high) Porocidaris 



