THE OPALINID CILIATE INFUSORIAKS. 363 



Cretaceous (fig. 234) before the genus OpaUna was evolved in Asia 

 and to northern South America in the latter part of the same period 

 by way of the Siberia-Alaska-California-Ecuador ridge and have 

 entered North America later (fig. 237), passing to Central America 

 and the West Indies in the later ^Tertiary. 



As to South America. Both Bufo and a number of Equatorian 

 genera of Bufonidae are now in northern South America. None of 

 them contain any multinucleated Opalinids, so Bufo can not have 

 entered South America from North America and Central America 

 unless it came before Cepedea and Opalina w^ere in North America 

 and Central America. Apparently Cepedea was not in continental 

 Asia until the middle-Tertiary (fig. 237) when Asia and India came 

 together. Opalina was probably evolved in Asia from Cepedea. 

 Bufo may, therefore, have evolved in southeastern Asia during the 

 Cretaceous (fig. 234) and have spread during the same period to 

 South America by the Alaska-California-Ecuador ridge. It could 

 hardly have come to South America by way of North America and 

 Central America, for the Central American route was not open until 

 the later Tertiary (figs. 236, 237) and by that time Cepedea and 

 Opalina were doubtless present in the North American and Central 

 American toads. If the South American Bufos came from either 

 North America or Asia during the late Tertiary they would now bear 

 Cepedea and Opalina^ but they do not. Bufo^ therefore, either came 

 to South America in the Cretaceous by the Alaska-Ecuador ridge, 

 or it evolved in northwestern South America, Ecuador, from some 

 genus of Equatorian Bufonid already there, and migrated to Euro- 

 Asia traversing the Ecuador- Alaska ridge in a northward direction. 

 I do not see that our data show which of the two places of origin, 

 Malaysia or northwestern South America, is the more probable. 



In either case, Bufo apparently did not enter North America 

 directly from South America, for there was no open route by way of 

 Central America until the later Tertiary when Zelleriella had reached 

 tropical America, and North American Bufos do not contain ZeZ- 

 leriella, the characteristic Opalinid parasite of the South American 

 toads. Bufo apparently came to North America from Asia during 

 the Tertiary, traveling by way of Alaska. The Cretaceous Alaska- 

 Ecuador ridge was occupied by Bufos and some of these may also 

 have passed eastward later to middle North America. 



Bufo, then, apparently arose in the Cretaceous after Australia had 

 parted from Euro- Asia, evolving either in southeastern Asia or in 

 northwestern South America. It entered North America from the 

 west, not the south. There apparently has been no direct spreading 

 of Bufos from North America to South America or the reverse. 

 Bufo seemingly entered tropical Africa from Euro-Asia by way of 



