SKELETON OF THE CROCODILE. THE CRANIUM. 249 



the ventral side the two alisphenoids are seen to almost 

 or quite meet one another immediately below the frontal, 

 and then to diverge, forming an irregular opening partially 

 closed by cartilage in the fresh specimen, through which 

 the optic nerves leave the cranial cavity. Further back the 

 alisphenoids meet one another for a narrow area, and then 

 diverge again, so that between each and the rostrum of 

 the basisphenoid there appears an opening (fig. 44, III, YI) 

 through which the oculomotor and abducens nerves leave 

 the cranium. Further back still each is united for a short 

 space with the basisphenoid, pterygoid and quadrate, and 

 then becomes separated from the quadrate by a large foramen, 

 the foramen ovale (fig. 44, Y), through which the whole of 

 the trigeminal nerve passes out. 



The dorsal portion of the parietal segment is formed by 

 the parietal (fig. 45, 4), which though double in the embryo, 

 early comes to form a single bone. It extends over the 

 posterior part of the cranial cavity, and is continuous in front 

 with the frontal, behind with the supra-occipital, and laterally 

 with the postfrontals, squamosals, alisphenoids, pro-otics and 

 epi-otics. It forms the inner boundary of a large rounded 

 vacuity on the roof of the skull, the supratemporal fossa. 



The frontal segment is very imperfectly ossified, there 

 being no certain representatives of either the ventral member, 

 the presphenoid, or the lateral members, the orbitosphenoids. 

 On the dorsal side there is, however, a large development 

 of membrane bones. There is a large frontal (fig. 45, 3), 

 unpaired, except in the embryo, united behind with the 

 parietal and postfrontal, and drawn out in front into a long 

 process which is overlapped by the prefrontals and posterior 

 part of the nasals. The frontal ends off freely below, owing 

 to the orbitosphenoidal region being unossified, it forms a 

 considerable part of the roof of the cranial cavity, but takes 

 no part in the formation of the wall. 



Each prefrontal (fig. 45, 14) forms part of the inner wall 



