248 



THE VOYAGE OF H.M.S. CHALLENGER. 



L.+ Meoma, T. 

 L.++ Schizaster, T. 

 L.+ Metalia, T. 

 C."^ Neolampas. 

 L. "*""*" Briiisopsis. 

 L.++ Hemiaster, C. 

 L."*""*" Echinocardium, 1 

 L.+ Spatangus, T. 

 L.++ Maretia, T. 

 L."*" Echinolampas, T. 

 L.+ Mellita, recent. 

 L."*" Peronella, recent. 

 L.+ Clyp)easter, T. 

 L."^ Echinanthus, T. 

 L. "*"■*" Echmocyamus, C. 

 L.++ Fibularia, C. 

 L.+ Toxopneiistes, T. 



L.+ Hipponoc, T. 



L.++ Echinus, C. 



L.+ Salmacis, T. 



C.""" Temnechinus, C 



L.+ Temnopleurus, T. 



L.+ SphcBrecliinus, 



L."*" Echinometra, T. 



C."*" Phormosoma (Echinothuri- 



d£e, C). 

 C."*" Micropyga. 

 C.+ Cielopleurus, T. {Magnosia, 



C). 

 L.+ Arhacia, T. 

 C."*" Salenia, C. 

 L.++ Gonioeidaris, recent. 

 L.+ aVZarts, T. 

 L. "'■"*' Dorocidaris, C. 



It is interesting to note that all the genera which have the greatest bathymetrical 

 range, which extend from the littoral to the abyssal region, are at the same time genera 

 which date back to the Cretaceous, while those which have a somewhat more limited 

 range date back to the Tertiaries, and those genera which happen to extend only slightly 

 beyond the strictly littoral range date back only to the more recent Tertiary periods. Of 

 com-se the difficulty of tracing the connection between the species of the present epoch, 

 which may have ranged in the Tertiary, in shallow seas, or in deeper water is very great, 

 and the mixture thus created in the littoral fauna of the present clay it is practically 

 impossible to disentangle at present, if we take into account the impossibility of 

 determining what are strictly deep-sea genera at the present day on account of the great 

 bathymetrical range of many genera, and the possibility that what may be to-day a 

 littoral species may have been a deep-water genus in older geological times or vice versa. 



The extremes of temperature which we find in the sea at different depths character- 

 ising the different l.)athymetrical regions we have recognised are much smaller than the 

 extremes of temperature which characterise our terrestrial fauna and flora. We have 

 no such extremes as are distinguished on land between a tropical and an arctic fauna 

 or between a fauna in the tropics near the level of the sea and one near the lower limit 

 of perpetual snow. Yet in the one case the difference in pressure of the surrounding 

 medium is small, say at the outside there is not a greater difference than two-thirds, 

 while between the abyssal regions and the littoral regions we have no such extreme of 

 temperatures, but extremes of pressure represented by a ratio of one to three hundred 



