152 



THE ANATOMY OF VERTEBRATED ANIMALS. 



the Teleostei y and of two neural arches, which lie over its 

 anterior end, and become anchylosed with it. The anterior 

 face of the coccyx usually presents two concave facets for 

 articulation with the posterior convexities of the sacrum. 



The cavity of the cranium is not narrowed anteriorly by 

 the development of an interorbital septum in any Amphibian. 

 All existing Amphibia have ex-occipitals developed in the 

 walls of the cartilaginous cranium ; but it is not certain that 

 any such ossifications existed in Archegosaurus, though they 

 are present in other Labyrinthodonts. 



No Amphibian possesses a complete basi-occipital, supra- 

 occipital, basisphenoid, alisphenoid, or presphenoid cartilage 

 bone. In existing Amphibia, a prootic ossification appears 

 to be verj'^ constant. The constant existence of distinct opis- 

 thotic and epiotic elements is doubtful. 



The Frog's skull is characterized by the development of a 

 very singular cartilage-bone, called by Cuvier the " os en 

 ceinture^'* or girdle-bone. This is an ossification which invades 

 the whole circumference of the cranium in the presphenoidal 

 and ethmoidal regions, and eventually assumes somewhat the 

 form of a dice-box, with one-half of its cavity divided by a 

 longitudinal partition. The latter, corresponding with the 

 front part of the bone, extends into the prefrontal processes 

 in some frogs, protects the hinder ends of the olfactory sacs, 

 and is perforated by the nasal division of the fifth nerve. The 

 septum, therefore, answers to the ethmoid, the anterior half 



Fig. 54. — The cartilaginous cranium of Rana eficulenta. A, from above; B, from below; 



2/, the " OS en ceintm-e," or girdle-bone. 



of the girdle-bone to the prefrontals, or part of them, and the 

 posterior half of the girdle-bone to the orbitosphenoids of 



