THE LIMBS OF AMPBIBIA. 



155 



answer to the bone (z) of the Frog, and to its qiiadrato-jugal. 

 Between the nostril and the maxilla, the nasal bone and the 

 premaxi'la, there is a bone which seems to be an ossification 

 of the cartilaginous ala 7iasi. Another bone nearly encircles 

 the orbit, and, as a supra- and postorbital bone, has no ana- 

 logue among existing Amphihia. The palatine bones sur- 

 round the posterior and outer margins of the posterior nares, 

 and then extend back on the inner side of the maxilla, in a 

 manner unlike any thing observed among other existing Am- 

 phibia, But in the Ldbyrintliodonta^ both this disposition 

 of the palatine and the complete roofing over of the skull by- 

 bone are repeated, and there is a postorbital bone. 





jS.ir. 



Fig, 56. — Side and upper views of the skull of Trematosaurus. The sculpture of the cra- 

 nial bones is not represented in the lower half of the upper view of the skull, in order to 

 show the sutures more distinctly. 



The Labyrinthodont skull is further characterized by the 

 development of distinct pointed epiotics, like those of fishes, 

 and of paired ossifications, which take the place of the supra- 

 occipital, as in many Qanoidei. In many Labj^rinthodonts 

 the articular element of the lower jaw is completely ossified. 



Archegosaurus possessed branchial arches when young, 



