456 



COMPARATIVE ANATOMY. 



indeed, many ossifications in the cranium of the Amphibia which 

 resemble those in the Dipnoi. 



The primordial cranium gives rise to a few bones only. In the 

 posterior primary region the exoccipitals alone are present (Fig. 246); 

 each of these gives rise to a condyle (co). The next region of the 

 auditory capsule presents large lateral processes, which are attached 

 more exteriorly to the hinder portion of the palato-quadrate. The 

 anterior portion of this segment is provided with an ossification, the 

 prootic. This contains the anterior portion only of the labyrinth, 

 the hinder portion of which is contained in the exoccipital ; it also 

 forms a foramen for the trigeminus. There are sometimes indica- 



tor!? 



Fig. 246. Skull of the Frog. A from above. B from below. C from behind. D from 

 the side. In A and B the covering bones are removed from the right half of the 

 cranium, so that the whole of the primordial cranium and its ossifications can be seen, 

 and, in A, the spaces in the roof of the cranial cavity. Pa Fr. Fronto-parietals. 

 Na Nasal. Bs Parasphcnoid. Ty Tympanic. Ft Pterygoid. Bl Palatine. Vo Vomer. 

 J Jugal. Mx Maxilla. Pas Premaxilla. o Exoccipital. Be Prootic. co Occipital 

 condyle. Co Columella, fo Fenestra ovalis. Exits of the nerves : Optic. Tr Tri- 

 geminus. Y'j Vagus. In the lower jaw : da Dentary. a Angular. Art Articular. 



tions of an epiotic. The fenestra ovalis is a cleft in the region of 

 the labyrinth, which is covered over by a small piece of bone. 



In the anterior portion of the orbital region there are sometimes 

 more or less extensive ossifications. They either embrace the lateral 

 wall only of the cranium (Siredon), or form a circular piece of bone, to 

 which Cuvier gave the name of " girdle-bone." This bone may 

 extend into the ethmoidal region, and even reach to the base of the 

 nasal capsules. 



The paired parietal s and frontal s are covering bones. In the 

 Anura these fuse together on either side to form a fronto-parietal 

 (Pa Fr). In front of these, and separated from one another by 

 the frontals, are the nasals, which we now meet with as constant 



