THE SKULLS OF BEPT1LIA AND AYES. 227 



The Crocodiles, on the other hand, possess a large and 

 distinct lateral ossification in front of each pro-otic (A S, Fig. 

 88, B). This ossification bounds the foramen for the third 

 division of the fifth nerve in front, and unites with the basi- 

 sphenoid below and with the parietal above, so far agreeing 

 with the alisphenoid. Since it extends so much further forward 

 than the alisphenoid ordinarily does, Cuvier has suggested 

 that it probably represents both the ali- and the orbito- 

 sphenoids ; but Stannius has pointed out the existence of two 

 small ossifications close to the optic foramina, with an in- 

 significant descending median stem at their bases. The former 

 he considers to be the orbito-sphenoicls and the latter the 

 presphenoid. 



In these Reptiles, in the Lacertilia and in the CJiehnia, the 

 basis cranii, as has been already stated, is modified anteriorly 

 into an inter-orbital septum, as in the Pike ; but in the Ophidia, 

 the Cyprinoid, or Batrachian, type of skull reappears, and the 

 cavity of the cranium is continued without any sudden narrow- 

 ing, from above downwards, from its posterior to its anterior 

 end. In the Ophidian skull (Fig. 90, B) the side walls of the 

 cranium, in front of the pro-otics, are completely closed in 

 by bones, which might readily be taken for alisphenoids and 

 orbito-sphenoids ; but, according to Rathke, they are merely 

 downward growths of the parietals and frontals, and therefore 

 can have nothing to do with the true lateral cranial elements. 



The anterior part of the basis cranii in Birds is always 

 vertically elongated into an inter-orbital septum, as in the Croco 

 dih'a, Lacertilia, and Chelonia. In the Ostrich (Fig. 88, A) the 

 presphenoid is completely ossified, but, in other members of 

 the class, the nature and extent of the presphenoidal ossifications 

 may vary greatly. The alisphenoid is always well ossified, and 

 occupies its characteristic position in front of the pro-otic and of 

 the exit of the third division of the trigeminal nerve (Fig. 

 88, A). The orbito-sphenoids, on the other hand, may or 

 may not be represented by bone. In the Ostrich they are 

 present, and are continuous with the presphenoid. 



Reptiles possess prefrontal and post -frontal bones, which 

 usually remain distinct throughout life, and are admitted to 



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