LIFE OF AMPHIBIANS. 609 



There are about 900 living species of Amphibia, most of them tail- 

 less. All are averse to salt water, hence their absence from almost all 

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

 are almost limited to the temperate parts of the northern hemisphere. 



History. It is likely that Amphibians were derived from a Piscine 

 stock related to the Dipnoi and perhaps also to the Crossopterygians. 

 The Stegocephali were the first pentadactyl animals (Lower Carboni- 

 ferous). Of living 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 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 con- 

 dition of these parts. But Mammals are, on the whole, more nearly 

 related to Reptiles. 



There are some remarkable affinities between the Stegocephali and 

 some of the extinct Reptiles, such as the Anomodonts, which in their 

 turn have affinities with Mammals. 



