CHAPTER IV. 



THE CLASS AMPHIBIA. 



The Ichthtopsida. — Class 11. — Amphibia. 



The only clearly diagnostic characters of this class as com- 

 pared with Fishes are the following : 



1. A/mphihia have no fin-rays. 



3. When limbs are present they contain the same skeletal 

 elements as those of the higher Vertebrata. 



Certain other structural peculiarities are common to the 

 whole of the Amphibia, and are very characteristic of them 

 without being diagnostic. Thus : 



1. The body is usually devoid of any exoskeleton, and 

 when scales, or scutes, are present in recent Amphibia, they 

 are concealed within the skin ( Ccecilia, JSphippifer). In the 

 extinct Labyrinthodonta, the dermal armor is confined to the 

 ventral region of the body. 



2. The vertebral centra are always represented by bone. 



3. The sacrum rarely consists of more than one vertebra, 

 though there are individual exceptions to this rule, as in 

 Menopoma. 



i. The suspensorial apparatus of the mandible is continu- 

 ous with the skull, which has two occipital condyles, and no 

 completely ossified basi-occipital. 



5. There are no sternal ribs. 



The Amphibia are divisible into the following groups : 

 A. A distinct and often long tail ; the vertebrae ampliicoelous or opis. 

 thocoelous ; the proximal elements of the tarsus not elongated. 

 A. Two or four limbs ; no scutes or scales. 



I. SaurobcUrachia or Urodela. 

 a. External branchiEc or gill-clefts persistent, or disappearinjj 

 only in advanced age ; no eyelids ; vertebrae ampbiccelous ; 

 uarpus and tarsus cartilaginous. 



