THE OPALINID CILIATE INFUSOKIANS. 359 



a distinct subfamily, Xenopodinae, or possibly might even be ele- 

 vated to family rank. The chief argument against regarding Pipa 

 and the African genera as closely related forms has been, as Gadow 

 (1909) states, their occurrence in regions so far apart and separated 

 by the Atlantic Ocean. But the resemblance between the genera is 

 such that most herpetologists have felt compelled to regard them as 

 related. The parasites of these genera would be of especial interest 

 in view of the problem of distribution. No specimens of HyTneno- 

 chirus have been available. Three preserved specimens of Xenojms 

 muelleri proved barren, so also did 22 specimens of Xenopus 

 laevis. Most of these were very stiff from preservation in too strong 

 alcohol and were in no condition for successful study. Three fine 

 specimens of Pipa from the United States National Museum were 

 opened, but with negative results. Nine specimens of this species 

 were sent me by the American Museum of Natural History, through 

 the kindness of Miss Dickerson, and these also proved uninfected. 

 Of 20 specimens of Xenopus calcaratus^ also from the American 

 Museum of Natural History, 6 were found to bear Protoopcdina 

 xenopodos, a species with a long, slender, posterior point, recalling 

 P. acuta of Australia and P. diplonucleata of Chile. The peculiar 

 breeding habits of Pipa pipa may account for its having no Ciliate 

 parasites, for the huge eggs and the larvae are carried upon the back 

 of the female, and, so far as known, the larvae do not have any free- 

 living, browsing, vegetarian stage in their development, the stage 

 during which other Anuran tadpoles ingest Opalinid cysts and be- 

 come infected. 



There are known among the Pipidae three living genera : Pipa^ in 

 northern South America ; Xenopus^ with three species in central and 

 southern Africa ; HyTnenochirus, from the same regions as XeTwpus; 

 and, besides these living genera, we know Palaeohatrachus., from mid- 

 Tertiary deposits in central Europe. In all likelihood the South 

 American representative of the family, or its ancestor, entered as an 

 immigrant from Africa by the trans- Atlantic route. The absence of 

 known fossil or modern Pipidae from Australia, southern South 

 America, Asia, and North America would argue in favor of an origin 

 of the family in Africa (fig. 233), with migration to northern South 

 America in the late Jurassic or early Cretaceous, and with migration 

 of one member of the family, Paleohatrachus^ from Africa to Europe 

 in the Middle Tertiary (fig. 237). It was apparently in early 

 ancestors of the Pipidae, in Equatoria, that the Opalinidae arose, as 

 early as Triassic times (p. 356) . The Pipidae seem to be a waning 

 family, and it is possible that they may have had a wider distribu- 

 tion in earlier times 



