340 THE AGE OF MAMMALS 



several members of the North American fauna have representatives as far 

 south as the Isthmus of Tehuantepec. 



It is true that the existing North American fish fauna is almost en- 

 tirely distinct from the tropical American fauna; the latter has its affinities 

 with the fishes of tropical Africa, and in Eigenmann's opinion necessitates 

 a former land connection between Africa and South America. Such a 

 mid- Atlantic land connection would l)e known as 'Atlantis,' and wiiile 

 of a highly hypothetical character, it is interesting to note that fresh evi- 

 dence in its favor has recently been l^rought forward bj^ J. W. Gregory.^ 

 This writer maintains that the striking similarity of the West Indian corals 

 to those of the Miocene deposits of the Mediterranean basin and to the 

 living genera of the Red Sea can onl}^ be exi^lained on the assumption 

 that there was a shallow water connection across the Central Atlantic at 

 a period no later than the Miocene. Moreover, this fauna could not have 

 come by way of the North because it is absent from the northern Miocene 

 of Europe and America. 



The geographic distribution of land mammals does not favor such an 

 hypothesis, although it is a convenient one for certain facts of distribution, 

 such as the occurrence of the water snake Pterosphenus in the Fayum and 

 in Eocene beds of Alabama, the distribution of the characines, cichlids, 

 siluroids, and probably of the octodont rodents. Likewise an archipelago 

 between western Africa and eastern America might have facilitated the 

 migration from Africa to America of the sirenians and the zeuglodonts in 

 Eocene times; but such migrations may equally well have occurred by 

 way of the Pacific coast line and through the gulf between the continents. 

 A south-Atlantic connection with Africa is quite another matter, which 

 has been discussed above under Antarctica (p. 75). 



Geologic Succession 



Our knowledge of the mammals of the Pliocene epoch in America is 

 very incomplete and still aw^aits the more active exploration and exact 

 research which have so nearly solved the mammalian succession of the 

 Miocene and earlier periods. The historic or geologic succession also re- 

 quires more exhaustive study. 



The formations which yield us vistas of Pliocene life in North America 

 are widely scattered, limited in extent, and less rich in complete foesil 

 remains than those of the Miocene. Despite the evidence afforded by the 

 invertebrate palaeontology of Florida, there is some doubt as to whether 

 certain of these older formations, here provisionally referred to the Plio- 

 cene, do not more properly belong in the Upper Miocene, where they have 

 been previously placed. 



Somewhat the same feeling prevails as to the age of the beds assigned 



' Gregory, J. W., Contributions to tlie Palaeontology and Physical Geology of the West 

 Indies. Quart. Jour. Geol. Soc, Vol. LI, no. 22, 1895, pp. 255-312. 



