HISTORY OF AMPHIBIANS. 467 



History. — It is likely that Amphibians were derived from 

 a stock from which the Dipnoi and perhaps also the Elasmo- 

 branchs sprang. The order Labyrinthodontia or Stego- 

 cephala 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 agree with 

 Mammals in having two condyles on the skuU ; it is only in 

 Amphibians that the articular element of the mandibular arch 

 remains cartilaginous, while the quadrate ossification is 

 small, and the squamosal extends down over it to the osseous 

 elements of the mandible, thus affording an easy transition 

 to the mammalian condition of those parts; the pectoral 

 girdle of Mammals is as much amphibian as it is saurop- 

 sidian ; the mammalian carpus is directly reducible to that 

 of Amphibians. 



There are many remarkable affinities between the Labyrin- 

 thodont Amphibians and an order 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. 



