564 CLARK : GEOGRAPHICAL RANGE IN RECENT CRINOIDES 



100 fathoms in the bathymetric range is correlated with an in- 

 crease of 213.33 square degrees in the geographical range. 



In the famihes confined to comparatively warm water the 

 geographical range is somewhat greater than the bathymetrical . 

 range, when 100 fathoms is considered as the equivalent of 213.33 

 square degrees, while in the families confined to cold water, and 

 the families represented in the polar regions, the reverse is the 

 case. This indicates that the curve representing the decrease 

 in area of the units measuring 15° on each side from the equator 

 to the poles is less marked than the curve representing the differ- 

 ence in the temperature between the surface water and that of 

 the abysses (which plays a very important part in the bathy- 

 metrical distribution of marine organisms) from the equator 

 to the poles. 



This method of comparing the bathymetrical and the geographi- 

 cal range of marine organisms which at no time during their 

 developmental history are pelagic takes no account of the land 

 masses within the geographical areas as calculated. These land 

 masses appear to be negligible; in other words, we appear to be 

 justified in considering them as everywhere potentially habit- 

 able by the crinoids occurring along their shores as far as the 

 meridians of longitude and the parallels of latitude to which these 

 crinoids now extend. 



The great tropical currents flowing northward, the Gulf Stream 

 and the Kuro-Siwo, do not act as distributors of crinoids as they 

 do of other types of organisms, for the reason that the littoral 

 forms which might be supposed to extend their range along the 

 shores washed by them, but which are confined within a very 

 limited range of temperature and of salinity, cannot survive 

 the conditions in the winter, when the currents in the northern- 

 most part of their course move southward and off shore, and 

 when further southward their inshore border is chilled and 

 freshened by drainage from the land. 



