80 CLARK: DISTRIBUTION OF CRINOIDS 



varying little from the surface to the bottom, as in the more typi- 

 cal parts of the Arctic, or, farther from shore, with an intermedi- 

 ate warmer layer, also as in certain parts of the Arctic ; while the 

 salinity is high, varying but little from the surface to the bot- 

 tom, as in the -Mediterranean. 



The Arctic and the Mediterranean are connected with the 

 Atlantic through geographically and bathymetrically restricted 

 channels. The Antarctic is connected with the oceans north of 

 it by a bathymetrically narrowly restricted thermal zone; for the 

 difference in temperature in the upper layers between the Antarc- 

 tic and the South Pacific, Indian and South Atlantic Oceans is 

 such as to preclude the entrance from any of the latter into the 

 former of all organisms excepting only those of the cold abysses. 



The Antarctic, therefore, is a great thermally isolated sea, 

 directly connected with the oceans to the north only through the 

 abysses, corresponding very closely in its physical, chemical and 

 biological characters to a geographically isolated sea which is 

 connected with the adjacent ocean only by a shallow strait. 



As the necessary corollary of oceanographic changes, all tend- 

 ing toward oceanographic simplicity, the more restricted an in- 

 land, or an isolated, sea becomes, the more featureless and the 

 more nearly vertical becomes the line representing the distribu- 

 tion of its crinoids in their relation to depth. Thus we are pre- 

 pared to find the line representing the bathymetrical distribu- 

 tion of the Arctic crinoids nearly vertical and almost straight, 

 although species of three quite different genera are involved. 

 Practically the same featureless vertical line represents the bathy- 

 metrical distribution of the crinoids of the Mediterranean, and 

 of the Okhotsk and Japanese Seas. 



The crinoid fauna of the Arctic Ocean includes two species 

 {Helio7netra glacialis and Ilycrinus carpenterii) apparently derived 

 from the crinoid fauna of the Bay of Bengal, and one (Hathro- 

 metra prolixa) derived from the Antarctic by way of the Atlantic. 



The Arctic fauna of the western part of the Sea of Okhotsk 

 and of the Sea of Japan includes one species {Heliomeira maxima) 

 derived from the Bay of Bengal, one -species {Thaumaiometra 

 tenuis) derived from the Pacific to the southward of Japan, and 



