94 REPORT — 1839. 



relatively larger than in the recent Sauria. This increase 

 of development of the anterior and posterior frontals is de- 

 pendent on the large size of the eye and the cavity destined to 

 contain it. 



In the composition of the facial part of the skull and the re- 

 lative sizes and disposition of the bones forming the nasal cavi- 

 ties and upper jaw, we have the same beautiful examples of a 

 transitional structure between the Crocodilian andLacertian types 

 of structure as have been noticed in other parts of the cranium. 

 With respect to the nasal apertures the tendency is mainly to- 

 wards the Lacertian structure, coupled with peculiarities purely 

 Ichthyosaurian. 



In the Monitors for example, the bony external nostrils com- 

 mence at the upper part of the cranium, at a very slight distance 

 in front of the orbit ; but they extend to near the anterior point 

 of the upper jaw, where they are bounded by the turbinated 

 bones. The rest of their circumference is due to the nasal, in- 

 termaxillary and superior maxillary bones. 



In the Crocodile, as is well known, the nostrils are placed at 

 the anterior part of the face, and are bounded by the nasal and 

 intermaxillary bones. In the Ichthyosaurus the nostrils are li- 

 mited to the position at which they commence in the Monitors, 

 viz. at a short distance anterior to the orbits ; and nearly the 

 whole of their posterior circumference is due to the lachrymal 

 bones, which do not at all enter into the composition of the 

 external nostrils in the existing Saurians. 



The characteristic structure and position of the external nos- 

 trils in the Ichthyosauri dubiously hinted at by Home, were 

 afterwards admirably determined by Mr. Conybeare. 



The upper maxillary bones are remarkable in the Ichthyosauri 

 for their small size ; they contain rarely more than the posterior 

 third part of the dental series of their own side. In the Croco- 

 diles the superior maxillary bones have a much greater relative 

 extent, and contain generally three fourths of the dental series. 

 The relative size of the maxillary bones is still greater in the 

 Lacertians. It is in Fishes that we find the nearest resem- 

 blance to the Ichthyosauri m the comparatively insignificant 

 share which the superior maxillary bones contribute to the 

 formation of the dentigerous margin of the upper jaw. The in- 

 termaxillary bones on the other hand present in the Ichthyo- 

 saurus as peculiar a degree of superior magnitude ; a difference, 

 however, which does not so much arise from the prolonged form 

 of the snout, as from the disproportionate shortness of the max- 

 illary bones. 



When we compare for example the jaws of the Ich. tenui- 



