2 THE VOYAGE OF H.M.S. CHALLENGER. 



anatomy of the Order would have delayed this Eeport far beyond the time allotted to me, 

 and it was taken for granted that this jaart of the subject would have to be undertaken 

 de novo for the whole group. The comparisons which have been attempted here are rarely 

 made from the study of monographs alone; they are based on an extensive acquaintance, not 

 only with the recent species but also with the fossil. The number of the latter, although 

 many times greater than that of the recent species, is yet by no means so great^ as to 

 make an accurate knowledge of all the principal species a difficult task. As it has been 

 my good fortune to examine for myself all but a few of the deep-sea species collected thus 

 far, this Eeport has been prepared under the best possible circumstances, with the aid 

 of the large collections both of recent and fossil species in the Museum of Comparative 

 Zoology at Cambridge, Mass. 



Having described the first important collection of deep-sea Echinids made, that of Count 

 Pourtales, it has been for me a most gratifying task to work up the report of the magnifi- 

 cent collection of the Challenger, and to have the opportunity, thanks to the generous 

 invitation of Sir Wyville Thomson, of going over the whole group of Echinids again with 

 so much new and important material at my command, which could not fail to develop 

 many novel and unexpected problems relating to the past and present history of Sea- 

 •urehins. 



Classification. 



With regard to the general classification of the Echinoidea, the additional light obtained 

 from the deep-sea genera regarding the systematic affinities of many fossil forms would 

 lead us to modify somewhat the systematic arrangements hitherto proposed. I have 

 already given my reasons for not adopting the artificial classification in vogue ; the 

 same objections which apply to the system adopted by Wright, apply with equal force to 

 those of de Loriol and of Zittel, wherever they are based upon characters of such 

 uncertain value as the presence or absence of teeth, or the presence or absence of actinal 

 cuts. We, of course, admit the ease of application of these characters as keys for the 

 identification of fossils, and also the difficulty we find in bringing fossil species within the 

 limits of the smaller subdivisions adopted among the recent Echinids, from the impossi- 

 bility of tracing ciiaracters generally imperfectly retained. Several of the sub-families 

 readily recognised among the recent Echinids are not generally adopted by palasontologists, 

 and they have, perhaps, in other directions divided the group into smaller subdivisions, 

 based on structural features which the study of recent tjrpes shows to have but little 

 value. 



I would in the first place regard the Palseechinoidea as one of the sub-orders of 

 Echinoidea, the others being the Desmosticha, the Clypeastridse, and the Petalosticha. 



' According to Zittel the number of fossil species does not exceed 2000. 



