96 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



plesiosaurs and most lizards (kionocraniate) ossification of the ascending process 

 of the quadrate forms an epipterygoid bone between the pterygoid and the 

 parietal. 



Membrane bones are more numerous than in the amphibians. In many 

 theriomorphs there is a supratemporal fossa between parietal and supratemporal 

 bones and the same is true of plesiosaurs, ichthyosaurs and chelonians. In the 

 rhynchocephals, dinosaurs, pterodactyls and crocodiles there is in addition, an 

 infratemporal fossa, bounded laterally by an arcade in which squamosal quad- 

 ratojugal and zygomatic participate in varying degrees. In the lizards the two 

 unite in a single temporal fossa by the disappearance of the upper arcade, and 

 lastly, in the snakes the lower arcade is lost and the fossa becomes a gap in the 

 side of the skull. 



Parietals and frontals are usually paired, a parietal foramen being common; 

 pre- and postfrontals usually occur, sometimes excluding the frontal from the 

 orbit. Lacrimals are common and the margins of the upper jaw are formed in 

 front by premaxilla and maxillary, the latter connected with the squamosal, 

 sometimes by zygomatic and quadratojugal, or the zygomatic may drop out, or 

 lastly the jaw may end with the maxillary. Several membrane bones may aid in 

 the formation of the roof of the mouth. There is a small parasphenoid in ich- 

 thyosaurs, plesiosaurs, many squamata, some rhynchocephals, and rarely in 

 turtles. It is usually associated with the basisphenoid and in ophidia it forms 

 the base of the interorbital septum. The vomers are paired except in the chelo- 

 nia, and only in Sphenodon of recent species do they bear teeth, and here but 

 one to each bone. The maxillaries usually have broad palatal processes extend- 

 ing toward the middle line, causing the choanae to open farther back, and in some, 

 these, together with the palatines and pterygoids, form a false palate, ventral to 

 the nasal passages, so that, as in the crocodiles, the choanae are carried far back 

 in the mouth. In some dinosaurs there is a rostral bone in front of the pre- 

 maxillae. 



The two halves of the lower jaw are united by ligament in most rhyncho- 

 cephals, snakes and pythonomorphs; by suture in crocodiles, rhynchocephals 

 and lizards; while they are fused in turtles and pterosaurs. All of the bones men- 

 tioned on page 78 may occur in the lower jaw, usually with distinct sutures, 

 while in crocodiles, theriomorphs and some dinosaurs there are gaps or vacuities 

 in its walls. In many dinosaurs there is a predentary bone at the tip of the jaw. 

 Except in the chelonia and a few isolated forms, both jaws bear teeth, which 

 may be restricted to maxillaries and premaxillaries, or may also occur on pala- 

 tines, vomers and pterygoids. In their fixation three types are found: acrodont, 

 when fused to the margin of the bone; pleurodont, when fastened to the side of 

 the bone; and thecodont, when implanted in sockets. 



The hyoid apparatus is much modified, but is adequately known only in 

 recent species. The branchial arches are usually better developed than the 

 hyoid proper, which is cartilaginous in most snakes and is lacking in the croco- 

 diles. In the chelonia (fig. 102) two branchial arches are usually present. 



The Theriomorphs (fig. 99) have a short, broad skull with parietal fora- 

 men; and that of the cotylosaurs was much like that of the stegocephals. In the 

 more dififerentiated groups the skull recalls that of mammals, especially in the 



