VENTRAL SKULL 5S5 



generally closely applied. All four of these may bear t(><'th. The 

 ectopterygoids are absent from modern tctnipods, hut the 

 pterygoids and palatines are usually fairly well (k'velopt'd. Tlie 

 latter tend to be placed horizontally to form the roof of the buccal 

 cavity, while the former tend to lie vertically and bound medially 

 the space containing the jaw muscles. Finally, outside these, 

 another series of dermal bones, the premaxilhe, maxilhe, jugals 

 and quadratojugals, form much of the functional jaw and 

 skeleton of the cheek region. It is to be noted that the quadrato- 

 jugal is not, as might be expected, a combination of quadrate 

 and jugal, but a bone running between those two. It is not present 



Fig. 456. — Lower jaw of Eogyrinus. Right half, inner surface. — From Watson, 



Philos. Trans., B. 1926. 214, 189. 



Ang., Angular ; Cor. I, U, and UI, coronoid bones ; P.Art., prearticular ; Po.Sp., post-splenial ; 



Sp., splenial. 



in mammals, where the jugal (also called malar or zygomatic) 

 joins the squamosal. The premaxillce are usually iirml\' sutured 

 to each other at the anterior end, but in snakes they are joined 

 only by an elastic ligament, which allows the mouth to stretch 

 in the swallowing of large prey. Premaxilla and maxilla generally 



bear teeth. 



The cartilage of the lower jaw usually ossifies onh' at the 

 posterior end, to form the articular, which is the part which arti- 

 culates with the quadrate, but in the Anura there is a small 

 ossification, the mentomeckelian, at the anterior end. Except 

 in the cartilaginous fishes the functional lower jaw consists 

 almost entirely of membrane bones, of which there may be several. 

 They are about at their maximum in Stegocephalia (Fig. 45O). 

 where, on the outside, there are a large dentary and smaller 

 coronoid, surangular and angular. All these curve over the edge 

 of the cartilage and are visible on the inside, where there are m 



