22 BULLETIN 75, UNITED STATES NATIONAL MUSEUM. 



nation why geographical isolation is not necessarily an important 

 factoi- among marine animals. Throughout the West Indian region 

 one of the commonest genera of brittle-stars is OpUocoma, of which 

 three well-marked species are found there. Two of these echinata 

 and riisei, are very nearly related and there can be no doubt that the 

 latter finds in the former its nearest ally. The differences between 

 the two species, while neither conspicuous nor of morphological 

 importance, are very constant and perfectly obvious. Yet the two 

 species occur not only on the same reefs and shores, but under the 

 same stones, where they mingle with each other, often in considerable 

 numbers. So far as the records, supplemented by the examination 

 of large series of specimens, hving and preserved, show, hybridization 

 between the two species is unknown, yet both are wholly oviparous 

 with external fertilization. The explanation of this curious. situation 

 was discovered by Grave in 1897," when he found that ecliinatahreeds 

 in late July and in August while riisei probably breeds in April and 

 May, as it had entirely finished its breeding season before the end of 

 June. We have here then a most striking case of "physiological 

 isolation," and geographical isolation is no longer a factor with the 

 two species concerned. 



To sum up the evidence here presented, it seems fair to say that, 

 in view of the large number of cases cited above where Jordan's law 

 does not seem to apply to ophiurans, geographical isolation has been a 

 less important factor in the specific differentiation of brittle-stars 

 than some form of physiological isolation. 



CONCLUSIONS AS TO DISTRIBUTION OF OPHIURANS IN NORTH PACIFIC OCEAN. 



1. Four distinct faunas combine to make up the North Pacific 

 group of species and these are designated as the Honshu, Bering, 

 Oceanic, and American. Of these the Honshu is the largest and 

 most diversified, wlule the Oceanic is the most widely distributed. 



2. Six species, well known from the North Atlantic and the seas 

 north of Europe, confirm the belief in a circumpolar fauna. 



3. The fine of division, on the Asiatic coast, between the Bering 

 and Honshu faunas, is not at either La Perouse or Tsugaru Strait, 

 but much farther south, at about the thirty-sixth parallel of latitude. 

 On the Pacific coast of Japan the line is apparently determined by 

 the meeting of the warm Kuroshino current with the cold current 

 from Bering Sea. The Sea of Japan appears to have been colonized 

 by the gradual influx of Bering species through the northern straits 

 and of Honshu species through the straits of Korea, the two groups 

 meeting in the southern half of the sea. 



4. About four-fifths of the species are shallow-water forms, only 

 forty-one being confined to water exceeding 300 fathoms in depth. 



a See Johns Hopkins-Univ. Circ, no. 137, 1898, p. 8. 



