ANATOMY OF BATRACHIANS. 467 



tinct Archegosaurus had in its larval life branchial arches, 

 and in fact so close are the affinities of some Amphibians to 

 the Ganoids that it is probable that both types have had a com- 

 mon origin ; while on the other hand the bones of certain 

 extinct scaly Labyrinthodonts have been regarded by some 

 authors as reptiles ; for example, the Carboniferous Mastodon- 

 saurus was described as a reptile, but has been referred to 

 the Amphibians by modern writers. 



The sternum or breast-bone (Fig. 429, s) first appears in 

 the Batrachians. The shoulder-girdle is in great part carti- 

 laginous. In the toads and 

 frogs (Anura) the fore limbs, 

 the radius, and ulna, and in 

 the hind limbs the tibia and 

 fibula, grow together ; there 

 are four toes in the fore feet, 

 and five toes in the hind feet. 

 In the Siren the hind legs are Fig . 429.-stemum and 



win finer in flip pono-o-snnkos of Frog (Rana ^nporaria). p, body of 



lll & ) J ' the sternum ; sc, scapula ; sc', supra-scap- 



( A tnnlninnn\ tlio limbs nvp nla ; co, coracold-bone, fused in the mid- 



(Ampmuma) , dle ]inc with its fe]low ?r the oppwitG 



either two or three-toed. side(); <rf, clavicle -,e, episternum. The 



extreme shaded double portion below p 



The teeth of modern Ba- is the sdphisteranm. The cartilaginous 

 . parts are shaded. After Gegenbaur. 



trachians are conical or lobate, 



and microscopically are simple, while those of the extinct 

 forms are mostly complicated by the labyrinthine infolding 

 of the walls, as seen in microscopic sections ; the teeth of 

 many Ganoids have a similar, though much simpler struc- 

 ture. They are usually of the same size, and may be ar- 

 ranged on projecting portions of different bones of the mouth, 

 i.e., the premaxillary, maxillary, mandibular, vomerine, pal- 

 atine, and pterygoid bones, as in fishes. In tadpoles and 

 in Siren the jaw-bones are encased in horny beaks like those 

 of turtles and birds. In many Labyrinthodonts two tusks 

 were developed 011 the palate. The nasal canal is much as 

 in the Dipnoan fish, the internal opening being situated in 

 the Perennibranchiates just within the soft margin of the 

 mouth. In the salamanders and frogs it is bordered with 

 firmer parts of the jaw. The labyrinth of the ear is large, 

 and the tympanum or drum of the ear is external, Am- 



