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 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 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 choanz 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 choane are carried far back in the mouth. In some dinosaurs 
there is a rostral bone in front of the premaxille. 
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 maxillaries 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 
