GENEEA AND GEOUPS OF THE ECHIKOIDEA. 281 



all dwellers in very deep water, and indeed P. Jeffreysi is the 

 only species with a moderate bathymetrical distribution. 



In 1883 Prof. Sven Loven published his great work on " Pour- 

 talesia, a Genus of Echinoidea" (Kongl. Svenska Vetenskaps- 

 Akad. Handlingar, Bd. xix.), and he had obtained specimens from 

 the Survey of the N. Atlantic by the steamer ' Viiringen ' and 

 from A. Agassiz and Sir "Wyv. Thomson. He especially investi- 

 gated P. Jeffreysi. 



The descriptions of what Loven very truthfully called " the 

 most extraordinary Echinoid hitherto known," by the authors 

 alluded to, are models of plain exactitude, and those of Sir 

 "Wyville Thomson are in his best style. There is, however, con- 

 siderable diversity of opinion amongst the three naturalists regard- 

 ing the alliances of the genus with extinct forms ; but now that 

 it can be said that the morphology of one of the species has been 

 described with wonderful accuracy by Loven, the palaeontologist 

 may pass his opiDion on the subject. 



There is a difficulty in associating all the species now classified 

 under Pourtalesia in one genus, which has been felt by Agassiz 

 and Loven, and will interest the advanced students of the 

 Echinoidea. 



Can Pourtalesia ceratopyga, P. Jeffreysi^ P. rosea, dxi^ P. mi- 

 randa, for instance, be placed in the same genus ? A. Agassiz 

 notices that he was disposed to divide the recognized species ; 

 but on the examination of the whole group he determined to let 

 them remain under one head. The propriety of this is a matter 

 for discussion on ordinary zoological principles. It must be 

 remembered that Loven has shown that there are morphological 

 differences between some of the eight species which he recog- 

 nizes to belong to Pourtalesia, which are without example in 

 any other genus. Yet it seems to be impossible, in his opinion , 

 to break up the genus in order to meet the variability of im- 

 portant structures. In fact he considers that the species seem 

 to be undergoing evolution of a remarkable kind. 



There are species now included in the genus (P. ceratopyga, 

 P. Jeffreysi) which have disconnected apical systems, and others 

 (P. 7'osea, P. miranda) which have them compact ; and in the 

 first group the postero-lateral interradia are in contact actinally 

 and abactinally in most of the species, but in one the contact 

 is abactinal only ; in the other group there is not this contact. 

 Yet the structural affinities of the two groups are otherwise so 



