CHAPTER IX. 

 CLASS II. AMPHIBIA 1 . 



AMPHIBIA differ markedly from Pisces in the fact that 

 in the more abundant and familiar forms the skin is naked, 

 and that when the integument is prolonged into median fins 

 they are devoid of fin-rays. The notochord may persist, but 

 bony vertebral centra are always developed. These are some- 

 times biconcave, sometimes procoelous, sometimes opistho- 

 coelous. There is only one sacral vertebra, except in rare 

 cases. The cartilaginous cranium persists to a considerable 

 extent but is more or less replaced by cartilage bone, and 

 overlain by membrane bone. The basi-occipital is not com- 

 pletely ossified, and the skull articulates with the vertebral 

 column by means of two occipital condyles formed by the 

 exoccipitals. 



There is a large parasphenoid, but there are no ossifications 

 in the basisphenoidal, presphenoidal, and alisphenoidal re- 

 gions. In most cases the epi-otics and opisthotics are ossified 

 continuously with the exoccipitals. 



The palato-pterygo-quadrate bar is firmly united to the 

 cranium, so the skull is autostylic. The palatines and 

 pterygoids are membrane bones. Teeth are nearly always 

 borne on the vomers and commonly on the maxillae and 



1 T. H. Huxley, Amphibia (Encyclopedia Britannica). 



