1 8 ANURA CHAP. 



The pterygoid bones are most fully developed, so as to reach 

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

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

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

 in AmpTiiuma and in Siren. 



The quadrates are directed forwards in Necturus, Proteus, and 

 Siren, 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 

 by the meeting of two corresponding processes from the 

 squamosal and frontal bones respectively. This bridge is rarely 

 bony (Salamandrina, Triton), mostly ligamentous ; apparently 

 a reminiscence of the Stegocephalous condition. The two pre- 

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

 Cryptobranchus, generally in adult Tritons. They are most 

 reduced, and are toothless, in Siren. 



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

 Typlilomolye, and Siren. Their posterior end is frequently free, 

 loosely connected by ligaments with the pterygoid in Crypto- 

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

 case either just touching it (Tylototriton), 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 ANURA 



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 column 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 



