302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



here being extratropical changes in temperature. True crocodiles, ap- 

 pearing first in the Cretaceous, still occur throughout the Tropics. 

 The gavials and alligators appear in the Miocene ; the gavials are now 

 confined to southern Asia, the alligators (with the caymans) to Amer- 

 ica, with one alligator in southern China. Among the fresh-water 

 turtles the soft-shelled type, appearing first in the Cretaceous, still 

 occurs in Asia, Africa, and America, as far north as Canada; the 

 Pelomedusidae are found in Africa and South America ; and the snake- 

 necked turtles in South America, Australia, and New Guinea. 



The amphibians occupy a very specialized habitat, essentially as 

 constant as the aquatic and much less variable than the terrestrial. 

 They differ from fishes, with which they are sometimes united under 

 the term Ichthyopsida, in being intolerant of salt water and as adults 

 living largely out of water in humid regions. Their very restricted 

 habitat seems to have limited their possibilities for evolution. No 

 fossil caecilians are known. Caecilians are now confined to South 

 America, the East Indies, and Ceylon, their distribution recalling 

 that of the tapirs. Frogs of the modern type appeared in the Jurassic 

 and salamanders in the Miocene. Frogs and toads occur everywhere 

 except in the extreme north, though toads are absent from Australia 

 and Madagascar. Frogs are especially well represented in South 

 America, but poorly represented in North America. The South 

 American frogs have many affinities with those of Africa, while others 

 are allied to those of Australia. 



The salamanders and their relatives, on the other hand, are almost 

 wholly confined to the temperate regions of the Northern Hemisphere, 

 especially North America, to which a dozen or more genera, including 

 the curious Siren and Amphiuma, are confined. Their distribution 

 recalls that of the North American fresh-water fishes, especially 

 the ganoids. Numerous instances of discontinuous distribution 

 suggest that the present distribution of the amphibians is in large 

 part a restriction of a once more or less world-wide distribution 

 that was attained very long ago. Examples are, the Pipidae, the 

 most aquatic of the frogs, found in South America and Africa ; the 

 tailed or ribbed frogs (Liopelmidae), found in New Zealand and 

 from northern California to British Columbia ; the Cryptobranchidae, 

 represented by the hellbender {Gryptobranchus) in eastern North 

 America and by the giant salamander {Megalobatrachus) in Japan 

 and China ; and the Necturidae, represented by several species in east- 

 ern North America and one in the caves of southwestern Austria. 



The fresh-water fishes occupy a habitat which in tropical and sub- 

 tropical regions is the least subject to change of any. So they might 

 be expected to present the greatest faunal uniformity, and to include 

 the largest number of very ancient types still persisting. In the 



