HISTORY. 559 



oceanic islands. The Anura are well-nigh cosmopolitan ; the Urodela 

 are limited to the temperate parts of the northern hemisphere. 



History. 



It is likely that Amphibians were derived from a stock 

 from which the Dipnoi and perhaps also the Elasmobranchs 

 sprang. The order Labyrinthodontia or Stegocephala does 

 not seem very homogeneous ; it perhaps includes two or 

 more distinct orders. Of extant forms, the Gymnophiona 

 are more old-fashioned than the others. The modern types 

 gradually appear in Tertiary times. Some of the extinct 

 forms were gigantic. 



Huxley has emphasised the following affinities between 

 Amphibians and Mammals : the Amphibia, like Mammals, 

 have two condyles on the skull ; the pectoral girdle of 

 Mammals is as much amphibian as it is sauropsidian ; 

 the mammalian carpus is directly reducible to that of 

 Amphibians. In Amphibians only does the articular 

 element of the mandibular arch remain cartilaginous ; the 

 quadrate ossification is small, and the squamosal extends 

 down over it to the osseous elements of the mandible, thus 

 affording easy transition to the mammalian condition of 

 these parts. 



There are many remarkable affinities between the Labyrin- 

 thodont Amphibians and a class of extinct Reptiles known 

 as Anomodontia, and as the latter have also many affinities 

 with Mammals, it is possible that both Mammals and Ano- 

 modonts diverged from an Amphibian stock. The strange 

 extinct Eotetrapoda of Credner seem to unite the Stego- 

 cephala to the Rhychocephalia, a class of Reptiles now 

 represented by the New Zealand " lizard " Sphenodon. 



eUn e ^-^ 



