GENEEA. AND GEOUPS OF THE ECHIlSrOIDEA.. 283 



oi Pourtdlesia with tlie Anancliytidae was, however, subsequently 

 noticed by the same naturalist ; and in his great work on the 

 'Challenger' Echini, p. 124, he followed Sir Wyv. Thomson and 

 placed them in a subfamily " Pourtalesise " of the family Spatan- 

 gidse. Nevertheless A. Agassiz did not consider that the compact 

 apical system of his type, P. miranda, militated against the 

 admission of other species with disconnected apical systems into 

 the genus Pourtalesia. In fact he appears to discredit the taxo- 

 nomic value hitherto placed upon the different apical arrange- 

 ments — compact, elongate, and disconnected. He states (p. 133), 

 " It is remarkable how great is the variation in the extent of the 

 separation of the bivium and trivium at the apical system in the 

 different species of the genus;" and, in describing P. rosea, 

 he remarked (' Challenger ' Eeport, p. 141) : — " This species 

 is also remarkable for not having, as in other species of the genus, 

 its apical system divided by the encroachment of the postero- 

 lateral ambulacra into a bivium and trivium." 



It appears that the existence of compact and disconnected 

 apical systems with the correlative interradial arrangements in 

 one family must be admitted to be consonant with Zoology. 

 Their occurrence in the same genus cannot be maintained, except 

 under most unusual conditions. 



It is indeed highly fortunate that the genera EcJiinocrepis and 

 Spatagocystis, which have been so carefully described by A. 

 Agassiz and Loven, should have been discovered, for they let a 

 flood of light into the classification of the species included in 

 Pourtalesia. They will be considered further on, but it is 

 advisable to state now that Spatagocystis is in alliance with the 

 species of Pourtalesia which have disconnected apical systems ; 

 and that EcJiinocrepis is allied to P. miranda and P. rosea with 

 compact apical systems. 



The definitions of the genus Pourtalesia will be found in the 

 ' Revision of the Echini,' and in the Eeport on the ' Challenger ' 

 Echinoidea ; and one was published by Sir "Wy ville Thomson in 

 the ' Phil. Trans.' The descriptions by Agassiz are positive and 

 also very comparative. 



The morphology has especially been studied by Loven, and 

 published in ' Pourtalesia.' The following definition has been 

 collected from the works of these authors. 



