218 ON THE STRUCTURE OF THE SKULL. 



surface to the axial, or " Meckel's cartilage," of the mandible. 

 In the young Tadpole, a line drawn from the mandibular articu- 

 lation to the auditory capsule makes an acute angle with the 

 basis cranii ; but, as age advances, the angle becomes more and 

 more open, until, in the adult Frog, it is obtuse (Fig. 86), the 

 articular surface for the mandible having passed far behind the 

 auditory capsule. Of course the width of the gape increases 

 pari passu with this rotation of the mandibular suspensor. 



A survey of the series of the Amphibia from the perenni- 

 branchiates upwards, shows, in a persistent form, those inclina- 

 tions of the suspensor which are transitory in the Frog. Thus 

 in the perennibranchiate Siren, Siredon, Proteus, and Meno- 

 branchus, the angle is acute ; in the Salamander and Salainan- 

 droid Menopoma, it is nearly a right angle ; while, in the Frogs 

 and Toads, and the ancient Labyrinthodonts, the angle is 

 obtuse. 



In the lower Amphibia there is no girdle bone, the orbito- 

 sphenoid and the prefrontals being usually represented by 

 distinct bones. The frontals are distinct from the parietals, 

 and the maxillary and pterygo-palatine arcades become im- 

 perfect. 



Some of the Frogs and the Coecilise — the snake-like apodal 

 Amphibia — have the cranial bones expanded and anchylosed 

 into a sort of shield, presenting apertures only for the orbits 

 and the nostrils ; a process which is carried still farther, by the 

 addition of bones not known to existing Amphibia, in the extinct 

 salamandroid members of the class, called Labyrinthodonts. 



