88 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



rate is firmly united to the squamosal and sometimes to other bones (monimostylic). 

 The pterygoids extend forward to the palatines. In the squamata and the ichthyo- 

 saurs pterygoids and palatines are widely separated in the middle line, but else- 

 where they are closely approximate, the pterygoids even meeting the basisphenoid. 

 In all except chelonians, some dinosaurs and the typhlophida an ectopterygoid 

 (os transversum) extends from pterygoid to maxilla, while in 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 f rentals 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 jugal and quadratojugal, or the jugal 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 ichthyosaurs, plesiosaurs, 

 many squamata, some rhynchocephals, and rarely in turtles. It is usually asso- 

 ciated with the basisphenoid and in ophidia it forms the base of the interorbital 

 septum. The vomers are paired except in the chelonia, and only in Sphenodon 

 of recent species do they bear teeth, and here but one on each bone. The maxil- 

 laries usually have broad palatal processes extending 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 premaxillae. 



The two halves of the lower jaw are united by ligament in most rhynchocephals, 

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

 while they are fused in turtles and pterosaurs. All of the bones mentioned on 

 page 71 may occur in the lower jaw, usually with distinct sutures, while in croco- 

 diles, 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 maxillaiies and premaxillaries, or may also occur on palatines, 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 the- 

 codont, when implanted in sockets. 



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



