336 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



the genus arose since early Cretaceous times, when Australasia 

 separated from Malaysia (fig. 234), but study of the distribution 

 of the several groups of species in the genus will give different indi- 

 cation, namely, that Cepedea evolved probably during the Jurassic 

 from group 9 of the species of Protooyalina^ or from similar forms, 

 and that its place of origin was probably India or some portion of 

 the India-Ceylon-Madagascar-Africa bridge (p. 346). The absence 

 of both Protoopalinu and Cepedea from northeastern North America 

 shows that there may be other factors than mere land connection 

 which influence the distribution of Opalinidae. Two-thirds of the 

 known species of Cepedea are from the Eastern Hemisphere, a fact 

 which is some indication that the genus arose in the east. This seems 

 the more true, since we have so scant data from southern Asia. 

 Probably the list of eastern species would be considerably increased 

 if we knew the southern Asian forms. Another indication of Asiatic 

 origin of Cepedea is the fact that the four species (1 Cepedea^ 3 Pro- 

 toopalmae) which intergrade between Protoopalina and Cepedeay 

 are all Asian or Malaysian (see the second paragraph following). 

 The evolution of the genus Cepedea also culminates in the east, in 

 C. longa and C. segnientata from eastern Asia and Malaysia. 



The absence of broad Opalinas from New England and the absence 

 of narrow Opalinas from European Kanas is of interest. Rana 

 tetriporaria of Europe (which bears the broad Opaliiia ranarum) and 

 R. syVoatica of New England (which carries the narrow Opalina 

 virguloidea) are very close relatives. The ancestor of one or the 

 other of them probably crossed between the two hemispheres by way 

 of the Greenland-Iceland North Atlantic bridge. But whichever 

 one so crossed did not carry its Opalina with it. It is possible, this 

 migration took place from New England to Europe before the 

 American Rana sylvatica had met a Hyla and adopted its narrow 

 Opalina. (The narrow Opalinae probably evolved in the Hylids.) 

 Hyla reached North America probably during the latter half of the 

 Pliocene. If the migration of the ancestor of Rana sylvatica to 

 Europe occurred before this, it would have had no narrow Opalina 

 to carry with it. This suggestion would date the Greenland-Iceland 

 bridge as early as the Middle Pliocene or earlier. 



But we can recognize several groups of related species within the 

 genus Cepedea and this allows us to analyze the distribution still 

 further. The subgeneric affinities of some species are doubtful, but 

 there are other species which so resemble one another as to form 

 fairl}^ well demarcated groups. 



I 



