60 VERTEBRATA. 



CLASS AMPHIBIA. 



The animals of tliis class were formerly grouped with the rep- 

 tiles. Their stronger afTmity is, however, with the fishes, making 

 with the latter, Huxley's province Ichthyopsida. 



Their most striking characteristic is the metamorphosis which 

 they undergo after leaving the egg. Like fishes, they develop gills 

 upon the visceral arches for a longer or shorter period, but unlike 

 most fishes their "air-bladder" changes to a lung in the adult, 

 and the gills disappear in the higher orders. 



In the structure of the skull and the limbs they compare with 

 the higher vertebrates. The skull is broad and flat, with two 

 occipital condyles; there is no distinct neck; the vertebrae are 

 concavo-convex (Frog), or convexo-concave (Pipa), or biconcave 

 (Siren), or differing in the same individual ; the ribs are absent 

 or only vertebral, never sternal. The legs may be four or two, 

 or wanting. They have no fixed type of external shape. The 

 teeth vary greatly, or may be lacking. The only dental charac- 

 ters unlike reptiles is the location of teeth on the vomer. 



The Amphibians may be roughly grouped under four orders. 

 Urodela (" tailed-amphibians," as salamanders); Anura (frogs and 

 toads) ; Ophiomorpha (caecelians) ; and the extinct Labyrintho- 

 donts. Prof. Cope makes a lai'ger number of orders. Prof. 

 Huxley thinks that the class originated from some stock common 

 to the Elasmobranchs, Ganoids and Dipnoi, and probably as early 

 as the Devonian age. Their earliest distinction was the penta- 

 dactyl type of limb, and footprints probably amphibian are found 

 in American subcarboniferous. 



Amphibians were abundant, and the characteristic animals of 

 the Carboniferous age, but remains are rare in America since the 

 Permian. The Tertiary forms are of modern type. The tailless 

 forms are more abundant now. 



ORDER LABYRINTHODONTIA (Stegocephali). 



These were Amphibians of large size, and sometimes gigantic, 

 one skull being three feet long. They abounded in the Carbon- 

 iferous and the Trias. The American remains yet found are not 

 large, but the footprints indicate the largest forms. 



