xiii PHYLUM CHORDATA 291 



Oriental regions, but absent in Australasia and the Pacific 

 Islands. The Anura are almost universally distributed, and are 

 abundant in all the greater zoo-geographical regions : they are, 

 however, represented in New Zealand only by a single species 

 (Liopelma hochstetteri) very locally distributed, and are absent in 

 most Oceanic islands, a fact due to the fatal effects of salt water 

 upon the eggs and embryos of Amphibia as well as upon the adult. 



Remains of Stegocephali are found in considerable abundance 

 from the Carboniferous to the Trias, and one genus extends into 

 the Lower Jurassic, after which period the order apparently became 

 extinct. The Urodela and Anura are not known until the Eocene, 

 and no fossil remains of Gymnophiona have been found. 



Mutual Relationships. The perennibranchiate Urodela are 

 undoubtedly the lowest of existing Amphibia ; they lead up, through 

 such forms as Amphiuma, with persistent gill-slits but deciduous 

 gills, to the Land Salamanders, in which a purely terrestrial form is 

 assumed. The Stegocephali exhibit a parallel series of modifications, 

 some of them being perennibranchiate, others caducibranchiate. 

 Their skull is more complex than that of the Urodela, but their 

 vertebral column never reaches the same degree of specialisation as 

 that of the Land Salamanders, and in some cases shows a lower 

 grade of organisation than in any existing Amphibia. Both in 

 their skeleton and in the distribution of their lateral sense-organs 

 they show some affinity with the Crossopterygii. The Anura 

 are a very specialised group : their development indicates their 

 derivation from branchiate tailed forms, but there is no palseonto- 

 logical evidence on this point. 



CLASS IV. EEPTILIA 



Reptiles, Birds, and Mammals are associated together as having 

 in common certain features in which they differ from lower 

 Vertebrates. The most important of these is the occurrence in all 

 three classes of certain embryonic membranes termed the amnion 

 and the allantois, to be described subsequently. The term Amniota 

 is, accordingly, frequently used for the group formed by these 

 three highest classes of the Vertebrata. 



The classes Reptilia and Aves are much more closely allied with 

 one another than either of them is with the Mammalia ; and the 

 two first are sometimes associated together under the title of 

 Sauropsida. The following are some of the most salient features 

 of the Sauropsida when compared with the other Vertebrates : 



The epidermis always gives rise to important and characteristic 

 exoskeletal structures in the form of scales or feathers ; the dermis 

 may or may not take part in the formation of an exoskeleton. 

 The skull is well ossified ; it never in the adult state contains a 

 distinct parasphenoid. There is a single occipital condyle borne 



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