86 ö. hOVÉN, UN roUKTALHöIA, A GENUS OF EClllNOlDEA. 



threc liiibitiits, lu. d. 3500 m.; P. cei'atopyjL>;a, tliree habitats, m. cl. 3800 iii.; P. rosea 

 at 4750 ui. ; Ecliiiiocropis cuiieata at 2900 m.; Spatagocystis Challeugcri, two liabitats, 

 Mj. d. 3250 m., tlic inediuui of tlieir average dcpths being 3800 inctrcs. Above tlic gene- 

 ral medium deptli of 2900 inetres and up to 442 m., from the Globigeriria ooze to 

 the saiidy mud mixed witli pebbles, were found two species: Pourtalesia Jeflreysi, in 

 foLir habitats, at a mean depth of 1300 metres; P. rairanda, two habitats, m. d. 1200 

 m.; and one species alone, Pourtalesia laguncula, from 630 m., sandy mud, 5° C, 

 down to 5303 m., red clay, 1° C, in five habitats, mean depth 3000 metres. At one 

 localit}', in the Antaretic ocean, half-way between the Cape and Kerguelen Island, at 

 a depth 2926 m., four species were found living together at one locality, Pourtalesia 

 hispida, P. carinata, Echinocrepis cuneata, Spatagocystis Challengeri; half-way between 

 Kerguelen Island and South Australia, in one locality, depth 3576 m., three species, P. 

 ceratopyga, P. carinata, Sp. Challengeri were found with one another; two species, P. 

 phiale and P. hispida not far from there, at more tlian 62° S., depth 3611 m.; and 

 two others, P. cei'atopyga and P. carinata near the coast of Chile, at 4069 lu. The 

 rest were found singie. 



Thus, as far as our present knowledge goes, three species are Atlantic, and one 

 of these also Antaretic; two were found in the Pacific and five in the Antaretic Sea; 

 and of the ten species hitherto known none comes nearer to the surfaee than by 442 

 metres, while two descend to 1000 and to 2000 m., one to near 3000 m., three to 

 between 3000 and 4000 m., three to between 4000 and 5000 m., and one to beyond 

 5000 metres. 



The littoral region comprises the favoured zones of the sea, where light and 

 shade, a genial temperature, currents changeable in power and direction, a rich vegeta- 

 tion spread över extensive areas, abundance of food, of prey to allure, of enemies to 

 withstand or to evade, represent an infinitude of agents competent to call into play the 

 tendencies to vary, definite in kind and limited in number, which are embodied in each 

 species, and always ready by modifying its parts to respond to the influences of external 

 conditions. In this region the great majority of marine forms are at home, of the Echi- 

 noidea all the highest types, the Cidarida3, Echinida?, Clypeastrida?, Echinoneidte, Cassi- 

 dulida? and the Spatangidtc, Prymnadete and Prymnodesmian, and there live, with rare 

 exceptions, the recent representatives of the fossil types of preceding geological periods. 

 But not one among the known living species, not a singie fossil*) among the multitu- 

 des imbedded in the sediments of former seas, had suggested the possibility of a 

 combination of characters like that realised in the Pourtalesiada3, and science was not 

 aware of its existence, until, a few years ago, dredge and trawl descended into the 

 väst regions of the great clepths, where life eudures on härd terms, far beyond the 



') Al. Agassiz, in the list of known recent species, llep. Cliall. Echinoidea, p. 208, doubtingly adduces 

 as possibly a Tertiavy representative of the Pourtalesiadfe, the fraument described by Edw. Eorbes as 

 Echinarachuius Woodii, Echinoderraata of the British Tertiaries, Palieontographical Society, 1852, p. 12, 

 pl. II, tig. 6 a, 6 b. Professor P. JErFREY Bell has had the kindness to examine for me the original 

 specimeu of Eorbes, now in tlie British Museum. There seeras to be no reason whatever for regarding 

 it as having being part of something like a Pourtalesia. I arrived at the same concliision from the in- 

 spection of another fragment, also from Crag, lent me by Mr Eobert Bell, Chiswick. 



