EEPORT ON THE ECHIXOIDEA. 247 



Phormosoma, Pourtalesia, Pcdceotropus, Homolampas, Hemiaster, Aceste, A'erope, 

 Cystechinus, and Urechimis; while we find as eminently characteristic of the Southern 

 Ocean such strange forms of Pourtalesise as Spatagocystis, Echinocrepis, and Genico- 

 patagus, which may hereafter be found to extend north into both the Atlantic and Pacific, 

 if we can judge of the extension of the few species of Pourtalesia, Cystechinus, and 

 Urechinus into the Atlantic and Pacific from their numerous representatives in the 

 Southern and Antarctic Oceans ; while thus far as strictly Pacific we have only of the 

 abyssal species Cionobnssus and Argopatagus, and as strictly Atlantic only Calymne, 

 and perhaps Pygaster, showing from the bathymetrical range that the abyssal species proper 

 are few in number, are mainly limited to the Southern Ocean, and extend northward both 

 into the Atlantic and Pacific realms. That the continental species form no such restricted 

 littoral faunae as are characteristic of the species, having a narrow bathymetrical range, 

 but that we have as it were an Atlantic and a Pacific realm, which we are perhaps 

 justified in considering as off-shoots of the great separation w^hich took place, dividing the 

 great Southern Ocean when it extended uninterruptedly over the whole Southern Hemis- 

 phere, or at any rate when South America separated the Atlantic from the Pacific only 

 as a large island or an archipelago, geographical off-shoots of a time when the genera 

 characteristic of these two great realms may have been represented in the Atlantic and 

 Pacific Gulfs (to use Thomson's happy terminology) by identical species ; these now, in our 

 present epoch, bear to each other much the same relationship which the littoral species on 

 the two sides of the isthmus bear to one another, and measure as in that case the degree 

 of change or time which has elapsed since the separation has taken place, resulting in a 

 condition of things making a separation of the deep-sea forms into an Atlantic and a Pacific 

 continental realm possible. Just as the subsequent farther isolation of districts in the 

 Atlantic and Pacific may gradually have brought about a centralisation into littoral faunae 

 such as are generally recognised; while the species which have a great bathymetrical or 

 geographical range are those which have escaped the influence of these changes, some of 

 them which extend from the littoral to the aliyssal going back to the time when the 

 Atlantic and Pacific realms were not yet isolated, and thus explaining the existence of 

 the same species at distant geographical points, while others extending only to the 

 continental range from the littoral, recall the time when the Atlantic and Pacific realms 

 were already separated, and when the species of the continental range appeared as 

 representative species in the Atlantic and Pacific ; while those species which extend from 

 the continental to the abyssal have never been subject to the influences which have gone 

 to form either the continental ranges alone, or the littoral range. The genera wliich have 

 this great bathymetrical range are the following : I have marked witli T. genera which 

 occur in the Tertiary, and with C. those which occur in the Cretaceous. 



