OPALINID CILIATE INFUSORIANS — METCALF 603 



most edge of South America. Cepedea and especially Opalina are 

 the characteristic parasites. 



The family, other than Rana, is tropical and east Asian in distribu- 

 tion and probably arose in some palaeotropical region before Africa 

 and Madagascar separated, that is, before the mid-Cretaceous. The 

 distribution of these ranids other than Rana is somewhat similar to 

 that of the bufonids other than Bufo, except that the archaic bufonids 

 are lacldng in Madagascar and Papua and present in Australia, wliile 

 the older (?) ranids are absent only from Australia. Rana seems to 

 have been comparatively recently evolved, as is indicated by its 

 representation in both Australia and South America only in the 

 northernmost portions. Rana arose apparently in lands north of 

 Australia and South America. It evidently entered Papua during 

 the Tertiary period when Papua and Austraha had become perma- 

 nently separated. It probably arose in the Old World and entered 

 America by way of the Siberia- Alaska route in Tertiary times reach- 

 ing Central America before the Isthmus of Panama was formed (mid- 

 Pliocene) and after the Cretaceous East Pacific land-strip had so 

 changed as no longer to form a route to South America. Since the 

 mid-Pliocene neither Rana nor Bufo has used the Isthmus with free- 

 dom for southward migration from Central America, for Opalina 

 parasites, abundant in them in Central America, have not crossed 

 the Isthmus. Rana carries both Cepedea and Opalina, especially the 

 latter. It is a most vigorous genus and enters all lands to which the 

 way is open. It probably reached Papua late in the Tertiary and 

 from there passed by some accidental circumstance, very recently, 

 across the narrow channel to the northernmost tip of Australia, Cape 

 York, and has not been there long enough to spread to the south. It 

 is abundant in North America and well represented in Central Amer- 

 ica. It seems strange that since the middle PUocene, when the 

 Isthmus was formed, it has not used it more freely for a bridge to 

 South America. Only one species, R. palmipes, and possibly two 

 other ranids, Prostherapis and Phyllobates, got across, spreading only 

 a little way southward. » A geologic period and a half. Pleistocene 

 plus late Pliocene, would seem enough to allow many ranids to cross 

 southward, but neither the ranids nor Bufo have m.ade use of this 

 bridge, except for the one Rana and perhaps the two ranids men- 

 tioned. We know that Bv/o did not cross going southward, for its 

 opalinids are not found south of the Isthmus. At the same time that 

 all bufos and almost all ranids were refusing to pass southward over 

 the Istlimus of Panama, hylas and leptodactylids in abundance were 

 going northward across this bridge. So far as I can see we have no 

 data from hosts or parasites or geologic conditions or clmiatic mflu- 



' It is likely that ProMerapis and Phyclobate, entered South America with Dendrobatis. coming from 

 Africa by a trans-Atlant-c route (fig. 143. a). 



