BRISSINA. 585 



ERISSINA. 



Subfamily Brissina Gray, 1855. 



The plastrons formed by the fascioles are more numerous in this family 

 than in the other subfamilies of Spatangidae. We sometimes find well de- 

 veloped anal, subanal, as well as peripetalous and lateral fascioles occurring 

 together. The petals are usually equally developed, and the abactinal sys- 

 tem elongate, more or less sunken, with narrow median interambulacral 

 spaces, generally narrower than the poriferous zones, and covered only by 

 miliaries. 



HExMIASTER. 



Hemiaster Des., 1847, Ag. C. R. Ann. Sc. Nat., VII. 



Troschel was the first to call attention to the identity of structure of some 

 of the species of Tripylus, as established by Philippi, with Hemiaster, and to 

 suggest the propriety of uniting two of its species with Hemiaster. He 

 was prevented, however, from carrying out his suggestion by laying too 

 much stress upon the variation of structure of the anal lateral fasciole, — 

 structural features which we now know, from the study of the youno - of 

 several Spatangoids, to furnish no valuable generic characters. The peri- 

 petalous and subanal fascioles are the only ones which are subject to changes 

 of shape and not of quantity during their growth, and from their construc- 

 tion we may form generic divisions, taken in connection with other struc- 

 tural features. This has led me to consider many of the genera recognized 

 as simply subgeneric sections of Hemiaster — which are well and sharply 

 marked from our present stand-point, but which are based upon altogether 

 too limited materials — to be nothing more than temporary subdivisions. 

 The subgenus may be characterized by the presence of a peripetalous 

 fasciole, with more or less sunken petals, or shallow anterior groove ; the 

 presence of an abactinal subanal fasciole forms the subgenus Brissopsis ; 

 a subanal fasciole with anal branches, and a well-developed actinal plastron, 

 characterizes the subgenus Rhynobrissus. With a lateral fasciole, as in 

 Agassizia and Schizaster, and deeply sunken lateral ambulacra, the subgenus 

 Tripylus, as limited here, is formed. The ambulacral structure of Faorina, 

 combined with a simple peripetalous fasciole, and the addition of a well- 



