1 8 ANURA CHAP. 



The i)teiygoid l)ones are must fully developed, so as to. reach 

 the vomero-palatines, in the Amblystomatinae, in JVecfurus, and 

 iu Proteus ; they are reduced, so as to leave a gap, in Grypto- 

 hranchus, and still more in the Salamandrinae ; they are absent 

 in A^nphiuma and in Siren. 



The quadrates are directed forwards in Necturus, Proteus, and 

 f^iren, while in the other Urodela they extend transversely and 

 almost horizontally. The hyomandibular remnant, the so-called 

 operculum, is small, and forms a plate which fits into the fenestra 

 ovalis, extending as a ligamentous process upon the quadrate. 



The quadrato-jugal elements are reduced to ligaments. In 

 many Salamandrinae the large orbito-temporal space is divided 

 into an orbital and a temporal fossa by an arch which is formed 

 l)y the meeting of two corresponding processes from the 

 s(][uamosal and frontal bones respectively. This bridge is rarely 

 bony {SaJamandrina, Triton), mostly ligamentous ; — apparently 

 a reminiscence of the Stegocephalous condition. The two pre- 

 maxillary bones are lialAe to fuse into one, for instance in 

 Crijiitohraiiehus, generally in adult Tritons. They are most 

 reduced, and are toothless, in Siren. 



The two maxillary bones are absent only in Necturus, Proteus, 

 Typhlouiolge, and Siren. Tlieir posterior end is frequently free, 

 loosely connected by ligaments with the pterygoid in Crypto- 

 hranchus ; or with the distal portion of the quadrate, and in this 

 case either just touching it {Tiilototriton), or forming a broad 

 junction {Pacliytritoii). 



Each half of the lower jaw consists of a dentary, articular 

 and angulo - splenial. The splenial remains as a separate 

 element in Siren, ; in others only during the larval period. There 

 are no mento-Meckelian elements. 



Skeleton of the Axura 



The vertebral column. — The distinctive peculiarities of 

 the vertebrae of the Anura are that they are notocentrous, and 

 that about a dozen of them are modified and fused into an os 

 coccygeum. The whole colunni is the most specialised found in 

 the Vertebrata ; and various stages are rapidly hurried through 

 and obscured caenogenetically during the embryonic development. 

 Paired cartilages appear on the dorsal side of the thin chordal 

 sheath, and whilst tending to enclose the spinal cord in a 



