174 THE VERTEBRATE SKELETON. 



cartilage of the lower jaw and its investing membrane bones 

 generally have much the same relations as in bony fishes. 



URODELA. The skulls of the various Urodeles show an 

 interesting series of modifications and differ much from one 

 another, but all agree in the absence of the quadratojugals, 

 in the fact that the palatines lie parallel to the axis of the 

 cranium, and in the large size of the parasphenoid. 



The lower types Menobranchus, Siren, Proteus, and Am- 

 phiuma have longer and narrower skulls than do the higher 

 types. 



Menobranchus has a very low type of skull which remains 

 throughout life in much the same condition as that of a young 

 tadpole or larval salamander. The roof and floor of the 

 cranium internal to the membrane bones are formed of fibrous 

 tissue, not of well-developed cartilage. The epi-otic regions of 

 the skull are ossified, forming a pair of large bones which lie 

 external to, and distinct from, the exoccipitals. Proteus and 

 the Labyrinthodonts are the only other Amphibia which have 

 these elements separately ossified. The parietals send a pair 

 of long processes forwards along the sides of the frontals. 

 Nasals and maxillae are absent, as is likewise the case in 

 Proteus. Teeth are borne on the vomers, premaxillae, ptery- 

 goids, dentaries and angulo-splenials. The suspensorium is 

 forwardly directed. 



The skull of Siren resembles that of Menobranchus in 

 several respects, as in the forward direction of the suspen- 

 sorium and in the absence of maxillae, but differs in the 

 possession of nasals, in the toothless condition of the pre- 

 maxillae and dentaries, and in the fusion and dentigerous con- 

 dition of the vomers arid palatines. 



Amphiuma has a skull which, though narrow and elongated, 

 differs from those of Menobranchus, Proteus, and Siren, and 

 resembles those of higher types in the following respects : 



(1) the suspensorium projects nearly at right angles to 



