12 THE NECTURUS 



frontals along the inner edge. Posteriorly it articulates 

 with the exoccipital, laterally with the opisthotic and 

 prootic. The ventral face is marked by a distinct ridge 

 that extends along the outer edge. This ridge forms the 

 posterior wall of the braincase and articulates with the 

 parasphenoid. This wall takes the place of the orbito- 

 sphenoid. 



Parasphenoid 



The parasphenoid is a flat plate of bone that forms the 

 roof of the mouth and the ventral wall of the braincase. 

 The ventral surface is flat while the dorsal surface has a 

 pair of ridges that articulate with the frontals and parie- 

 tals. The bone is wide at the posterior end and narrow at 

 the anterior. The ventral face articulates with the vomers 

 at the exterior end by a flat, contact articulation. Dorsally 

 the bone articulates with the frontals by the ridge already 

 mentioned, with the prootic and with the exoccipitals at 

 the posterior end. There is no indication of the usual de- 

 pression for the hypophysis, the pit usually known as the 

 sella turcica of the higher vertebrates. This bone forms 

 a large, single, median plate with no separation on the 

 median line. 



Mandible 



The mandible of Necturus consists of three bones, the 

 dentary, splenial and angular. The articular is cartilage. 



The dentary is a large element, forming most of the 

 jaw. There is a symphysis at the anterior end of the pair 

 which is fairly firm. The upper edge bears a number of 

 sharp, peglike teeth. Practically the whole of the outer 

 surface of the jaw is formed by this bone. 



The angular is on the inner side of the jaw and forms 

 the posterior part of the inner surface. It sends a long, 

 thin projection anteriorly along the dentary. It enlarges 



