A. H. CLARK : THE" CRINOIDS OF THE INDIAN OCEAN. 19 



The fauna of southern Japan is apparently younger than that of the West 

 Indies. 



The central sea connecting the Bay of Bengal with central Europe had an 

 arm stretching northward across Russia. Certain adaptable genera, becoming 

 acclimated, followed this arm northward and gave rise to the present arctic fauna. 

 More recently one of these genera has spread southward over the north Atlantic. 



At a considerably later date a connection was formed whereby the East 

 Indian crinoids, becoming slowly acclimated, reached the antarctic regions. 

 There was also a connection between the antarctic regions and southern South 

 America, whereby these forms secured a foothold on the western coast of that 

 continent spreading rapidly northward to the Aleutian Islands (dipping downward 

 into deep water when passing under the tropics), and thence southward along the 

 east coast of Japan to Tokyo Bay. The antarctic fauna is apparently the 

 youngest of all the existing faunas, and the evidence of youth increases as we go 

 northward along the American coast. 



There are two significant facts in connection with the distribution of the 

 recent crinoids which should be noticed, though the data so far accumulated is not 

 sufficient to admit of definite and conclusive statements in regard to them. The 

 north Atlantic lacks certain deep water and intermediate forms which occur in 

 the south and central Atlantic, its fauna being composed of species all primarily 

 inhabitants of shallow water, though some of these deep-water and intermediate 

 forms have worked northward along the east American coast to Greenland. We 

 might infer from this that there was a land barrier across the mid-Atlantic at one 

 time and that the sea to the south of this barrier received its crinoids both from 

 the East Indian Httoral and from the deeper parts of the Indian Ocean, while the 

 sea to the north received only shallow-water species which came both from the 

 Mediterranean region and from the arctic. While the time since the complete 

 removal of this barrier has been insufficient for the southern forms everywhere to 

 extend their ranges into the north Atlantic, some, aided by currents, have been 

 able to do this in the western part. 



In the Pacific we find a similar condition. There is no continuity between 

 the north and the south except along the western coast of South and North 

 America, and in the abysses, the species in which might just as well have spread 

 from south to north in the eastern part as in the western. Apparently there was 

 a great tropical barrier, a continent or an archipelago lying in a shallow sea, which 

 prevented the northward extension of southern forms in the western Pacific 

 though this has been permitted in the eastern portion of that ocean. 



