III.] NORTHERN ORIGIN OF SOUTHERN FORMS. 131 



penguins (Spheniscidcz), which present a relation to other birds 

 somewhat analogous to that exhibited by the edentates to other 

 mammals, having no apparent affinity with any group -may prove 

 to be an exception to the rule of the northern origin of most of the 

 existing southern types of terrestrial vertebrates, since they are 

 quite unknown in the north, and occur fossil both in New Zealand 

 and Patagonia. ', / 



Another marked instance of the northern origin of southern 

 types is afforded by the side-necked, or pleurodiran Chelonia, 

 which although now restricted to the more southern parts of the 

 globe, were abundant during Secondary and early Tertiary times 

 throughout the northern hemisphere. A striking example of this 

 is shown in the family Pelomednsida, whose existing representatives 

 are confined to Africa, Madagascar, and South America. Among 

 these, two out of three genera, namely Sternothoerus and Pelomedusa 

 are found in Ethiopian Africa and Madagascar, one of them also 

 ranging into the Sinai tic peninsula ; while the third (Podocnemis} 

 has five species in South America and a sixth in Madagascar. 

 There occurs, however, in the upper Cretaceous of the United 

 States the allied extinct genus Bothremys, and Podocnemis itself is 

 represented in the London Clay and the Eocene of the Punjab. 

 Here the inference would seem to be that the latter genus 

 originated in the northern half of the Old World, passed by way of 

 India into Madagascar and Africa, and thence by a southern route 

 into Neoggea. Even if this particular genus occurred in the early 

 Eocene of North America, which it does not, it could scarcely have 

 crossed the sea into South America ; and the migration can hardly 

 have taken place since the union of the two continents. In 

 commenting on the distribution of Podocnemis Dr Blanford 1 

 observes that as the incursion of more modern types into Africa 

 appears to have driven out many of the older, it is in Madagascar 

 that traces of the relationship of the modern fauna to that of 

 Neogaea should be looked for. One such instance is the occur- 

 rence there of Podocnemis^ and a second that of the Centetidcz in 

 that island. Perhaps the occurrence of sucker-footed bats only in 

 Brazil where they are represented by the single species of 



1 Appendix, No. 8, p. 101. 



92 



