LIFE OF AMPHIBIANS. 609 
There are about goo 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.—lIt 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. 
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