CLARK: THE CIDAIUDAE. 193 



nearly uniform width except where madrepore juts in. 1 Primary spines some- 

 what flattened near base, conspicuously thorny ; collar wide, greenish, reddish 

 or dark with conspicuous white spots; in young specimens these white spots 

 project as granules, hut in mature specimens, collar is smooth ; actinal prim- 

 aries slightly curved, with a very wide collar, often more than half their length, 

 and provided with a distinct cap of outer layer of spine ; this cap is truncate, thick, 

 and somewhat serrate. Large globiferous pedicellariae are wanting in all available 

 specimens. 



Although there can be no doubt of the close relationship between this genus 

 and Phyllacanthus, the discovery of a new species of Stephanocidaris in the 

 Hawaiian Islands, of which numerous specimens are available for study, shows 

 how clearly justified A. Agassiz ('63) was in making Cidarites bispinosa Lamarck 

 the type of a separate genus. The characters shown by the primary spines are 

 exhibited in specimens only 12 mm. h. d., and even in these specimens the genital 

 plates are widely separated ; it is not, however, until a diameter of over 20 mm. has 

 been reached that the remarkable character of the abactinal system becomes ap- 

 parent. The three species here recognized are confined to the central and eastern 

 portions of the Indo-Pacific region. The following key is based on the examination 

 of 106 specimens of the first and third species. 



1 It is worth noting that in a young Stephanocidaris 6 mm. in diameter, the 

 ocular plates are all excluded from the periproct, except that of the left posterior 

 ambulacrum, which barely touches an anal plate ; in a specimen 7 mm. in diam- 

 eter, the left posterior ocular is clearly in contact with the anal system and the 

 right posterior ocular barely touches it ; in a specimen 12 mm. in diameter, the two 

 posterior, and the left anterior oculars are all clearly in contact with, while the 

 odd anterior ocular barely touches, the periproct ; in another specimen of the same 

 size, all the oculars except the right anterior are clearly included ; in a specimen 

 14 mm. in diameter, and in all larger ones, all the oculars are broadly in contact with 

 the anal system. It seems to be true, therefore, of Stephanocidaris that the oculars 

 of the bivium come into contact with the periproct before those of the trivium do 

 and of the latter the right anterior ocular is the last to enter. Examination of a 

 series of young Cidaris tribuloides shows that the same course is followed in that 

 species, except that in one specimen the odd anterior ocular was excluded, while 

 the right anterior was no longer so. These facts are strikingly in accord with the 

 condition often found in Tretocidaris and always in Acanthocidaris, where the 

 right anterior ocular is the only one excluded. And I may add that in Arbacia 

 nigra and spatuligera, in adult specimens of which the posterior oculars, and often 

 the left anterior, are in contact with the anal system, the same course of entrance 

 of the oculars is followed; and while I have found a very few specimens in which 

 the odd anterior ocular is also insert, I have yet to find an Arbacia in which 

 the right anterior ocular is not excluded. The reason for this condition is not 



clear. 



vol. li. — no. 7 23 



