338 HAWAIIAN AND OTHER PACIFIC ECHINI. 



STRONGYLOCENTROTID^E Gregory. 



GENERAL CONSIDERATIONS. 



The real relationship of the polyporus Echini with the Echinidse can probably 

 be made more evident, if the forms with a circular ambitus are separated from 

 those in which one axis is elongated. I therefore agree with Jackson (1912, 

 Mem. Boston Soc. Nat. Hist., VII) when he follows Gregory in separating 

 Strongylocentrotus and its nearest allies from the Echinometridse and giving 

 them family rank. The group contains some thirty or more species which form 

 so complete a series from those nearest the Echinidse to those most highly spe- 

 cialized that it is exceedingly hard to arrange them in genera. Of course taking 

 some one character as the standard, artificial sections, called genera, can be 

 recognized but the associations thus formed are often valueless. The connection 

 of the family with the Echinidse through its lowest genus Paracentrotus is so 

 plain it will hardly be questioned, the resemblance of Paracentrotus agulhensis 

 to certain species of Echinus is so great that the M. C. Z. has received specimens, 

 labelled "Echinus gilchristi" from the describer of the latter species, himself! 

 The Strongylocentrotidse are widely distributed but it is a remarkable fact that 

 not a single species is known from the Caribbean region. Nearly one third of 

 the species are found in the northwestern Pacific, mostly in Japanese waters. 

 All of the species are littoral and only a few extend into water exceeding one 

 hundred fathoms. 



THE SPINES, PEDICELLARI^E, SPH^RIDIA, AND SPICULES. 

 Plates 94, figs. 1-33; 95, figs. 16-24. 



In the character of the spines, the Strongylocentrotidse show little indications 

 of advance over conditions found in the Echinidse. In most of the species the 

 spines are relatively short and there is no marked contrast between the primaries 

 and the other spines. In Echinostrephus however the abactinal primaries are 

 conspicuous; they are long, slender, very smooth, and rather sharp. In Helio- 

 cidaris and in some of the most specialized species of Strongylocentrotus, the 

 primary spines are long and stout, usually quite smooth, and more or less pointed; 

 in one species of Heliocidaris they are remarkably short and thick. The charac- 

 ter of the primary spines may thus be of considerable use for specific distinctions. 



