supraoccipitol 



P°^'^*°' postfrontal 



postorbital 



prefrontal 



exoccipital process' 

 occipital condyle 



basipterygoid process' 



squamosal 



posttemporol fenestra 



jugal 



paroccipitol process of opisthotic 



quadrate 



quadrotojugaj 



(fused to quadrate) 



cranioquadrate fissure 

 fenestra vestibuli 

 IX-X-XI 



epipterygoid 

 postorbital 

 postfrontal 



P°'''«*°' parietal foramen 



squamosal epipterygoid 



parietal 



prootic incisure 

 membranous cranial wo 

 basisphenoid 



B 



parasphenoid 



supraoccipitol 

 prootic prootic 



supratrigeminal process 

 facial toromen 

 poroccipital process 



internal carotid 

 ntercalore 

 quadrate 



parasphenoid 

 squamosal 



pterygoid 

 VII / / ^ — " basipterygoid process 



jugal quadratojugal 



ascending process 



temporal fenestra 

 ^supraoccipitol 



quadratojugal 

 basisphenoid 



Figure 4-8. Skull of Sphenodon. A, rear view; B, lateral view of cranium with temporal and labial 

 arches removed; C, medial view of right half of cranium. 



from the lateral nasal wall into the nasal passage. The basic 

 peculiarities of this type of skull suggest a relationship to the 

 diadectomorphs. The peculiarities of this type, and the dia- 

 dectomorphs described below, led Olson (1947) to divide 

 the reptiles into two subclasses, the Parareptilia and the 

 Eureptilia. 



Alligator The crocodilians are the sole remaining archo- 

 saur. They are a close-knit group and are quite similar in 

 most details. The American alligator and crocodile differ 

 m the width of the snout, in the contact of the nasal process 

 of the premaxilla with the nasal between the e.xternal nares 

 m the alligator, and in the disposition of the teeth. 



The alligator skull has a diapsid roof (Figure 4-10). The 

 superior temporal opening is small, and the posttemporal 

 fossa is reduced to a chink. A septomaxilla and epipterygoid 

 are lacking. The vomers do not contact or even approach 

 the prcmaxillae, a state related to the forward outgrowth of 

 the snout which left the vomers far behind (Figure 4-11). 

 There is a secondary palate, a parallelism to that of the mam- 



mal, which extends the nasal passages back to the area of 

 the glottis. Extensions of the maxilla, palatine, and ptery- 

 goid are involved in this secondary palate. Along with these 

 palatal changes, the pterygoids are moved backward and 

 medially, eliminating the basipterygoid processes and sutur- 

 ing with the basis cranii. As a result of this contact, the in- 

 ternal carotid artery and the palatine nerve become enclosed 

 in a "pterygobasipterygoid" canal. Bony eustachian tubes 

 are also formed. The parasphenoid has three centers in the 

 embryo but is largely obliterated by the palatal changes. 



The orbitosphenoid ossifies in the antotic pillar, then 

 spreads into the metotic pillar and the region of the 

 epipterygoid, where it achieves contact with the pterygoid. 

 Ossification follows the same pattern as chondrification. In 

 the lizard this bone arises in the metoptic pillar, and the 

 antotic pillar sometimes never forms. 



A sclerotic ring of bony plates is lacking. The conical teeth 

 are thecodont, that is, set in sockets like those of the mam- 

 mal, and are replaced by new teeth developing in the base 

 of the socket (see Figure 8-64). These teeth do not form a 



OTHER TETRAPODS • 71 



