THE OPAL.INID CILIATE INFUSORIANS. 373 



The distribution of the Anura and their Opalinids gives no evi- 

 dence of direct spreading in either direction across the Atlantic 

 Ocean between Europe and North America. The absence of Pro- 

 toopolina and Cepedea from New England is signicant in this con- 

 nection. 



We note that our data agree with the general belief that lines of 

 demarcation between northern and southern areas have had, on 

 the whole, more influence upon migration routes than have separa- 

 tions between eastern and western areas. Spreading has been more 

 along lines of longitude than along lines of latitude. Especially 

 emphasized have been the routes between Australasia and Pata- 

 gonia; between Australia, India, Africa, and South America; and 

 between North America and Siberia. North and south migration 

 between Australia and Asia has been restricted aj)parently chiefly 

 to the Jurassic. Since the beginning of Mesozoic times Africa 

 seemingly has had communication with the Euro-Asian lands to the 

 north of it in the early Triassic (India) , in the early Jurassic (India) 

 and since the middle of the Tertiary (Isthmus of Suez), but we have 

 little evidence of the passage of Anura between Africa and Euro- Asia 

 except in the later Tertiary. The Isthmus of Panama and Central 

 America as a whole has not been as freely used for migration by the 

 Opalinids and Anura as would seem natural. The Pipinae, Lepto- 

 dactylidae, Gastrophrynidae, Raninae and Dendrohatinae of South 

 America have not passed northward ; the Pelobatidae have not passed 

 from North America, into the southern continent, nor have any of the 

 Raninae except the one species Rana palmipes, the other genera of 

 Raninae present in South America never having been in North 

 America. The Pipinae seem to be decadent forms and so are of less 

 significance in this connection, but the Leptodactylidae and Gastro- 

 phrynidae are vigorous southern forms. The Dendrohatinae and the 

 distinctively South American genera of Raninae have not shown 

 much recent ability to spread, being still confined to their oM 

 northern South American habitat. The Pelobatidae, on the north, 

 are far from dominant forms, but they have spread widely in North 

 America. Rana, Bufo, and Hyla are the dominant genera of Anura 

 to-day. Wliy has Hyla been able to pass northward, while Rana and 

 Bufo have not entered South America from the north at least in 

 times since the genus Opalina was in America, except for the one 

 species, Rana palmipes, which indeed probably entered South 

 America after Opalina reached North America? Ranas and Bufos 

 are present in Central America and they bear both Opalinae latae 

 and Opalinae angustae. It is but a step across the Isthmus into 

 northwestern South America where general conditions are very 

 similar to those in Central America. Why have they not taken this 



