LITTORAL DISTRICTS. 223 



Tropical Atlantic District (PI. A). 



It would be highly interesting to have sufficient data on the bathymetrical 

 range of the species inhabiting the Atlantic to arrive, by a careful com- 

 parison of the species found on both sides, at some definite conclusion 

 regarding the influence which depth and temperature have upon the 

 distribution of species. Unfortunately, we can make but very limited 

 comparisons. In spite of the zealous investigations of the elder and 

 younger Sars on the coast of Norway, the Dredging Reports of McAndrew 

 and Barrett and of Nermann, and lastly the data furnished by the Porcupine 

 Expedition from the European coasts, we have on our own coasts such a lim- 

 ited range for comparison that any conclusions can be but hints for future 

 use. The identity of several species of the Echini from Florida and 

 the coast of Norway confirmed in a remarkable manner the suggestion first 

 made by Loven of the possibility of finding in the warmer seas, at great 

 depths, arctic species, the most striking confirmation being the existence 

 of Rhizocrinus at great depths off Florida, off the Azores, on the coast of 

 Portugal, and off Norway. Yet when we come to make a more detailed 

 comparison of the species found on both sides of the Atlantic, we find some 

 of them having such an extraordinary range in depth that, on either 

 side, the extremes were far beyond the effect of any influence, due either to 

 pressure or to temperature. Such were, for instance, Echinocyamus pusillus, 

 Echinocardium corclatum and ovatum ; while Brissopsis lyrifera, Dorocidaris 

 papillata, Echinus norvegicus, Asthenosoma hystrix, Pourtalesia, Homo- 

 lampas, and Schizaster fragilis occur only at such depth in the Straits of 

 Florida as show that temperature and not depth is the main agent in the 

 distribution of the species of Echini over the bottom of the Atlantic. The 

 temperature being dependent upon the action of the sun only to moderate 

 depth, it follows that we reach comparatively soon a depth below which 

 the temperature is comparatively uniform, near the zero point ; this uniform 

 tempera 'e probably extending over the greater part of the bottom of 

 the ocean, at depths varying with the latitude. The cold bottom connect- 

 ing opposite shores in the North Atlantic is probably inhabited only by 

 more or less arctic species, having an extraordinary wide geogra hical range, 

 and corresponding to the arctic character of the fauna and flora of mountain 

 chains and high summits. The only difference between the land and sea 

 being that the ranges of variations of temperature are so small, we cannot 



