44 THE SKELETON OF THE FKOG 



tube is imperfect, there being one large anterior fon- 

 tanelle, and two smaller posterior fontanelles, which 

 are closed by membrane only. In the cartilage are 

 developed cartilage-bones, and around it membrane- 

 bones. 



To study the aranium satisfaotorUy, the membrane-hones should 

 be stn'ipped from one of the skulls which you have prepared far 



r- 



Cartilage-bones of cranium. 



i. The exoccipitals are two irregular bony masses at 

 the sides of the posterior end of the skull. They 

 almost completely surround the foramen magnum 

 or entrance to the cranial cavity ; and bear on 

 their posterior surfaces the occipital condyles, 

 two oval convex processes which articulate with 

 the first vertebra or atlas. 



Fig. 8. — The frog's skull, from the ventral surface. 

 a, parasphenoid ; c, columella ; e, exoccipital ; /.p, frontoparietal ; 

 m, maxilla ; n, vomer ; e, pro-otic ; p, pterygoid ; fa, palatine ; 

 pm, premaxilla; y, " quadratojugal " ; se, sphenethmoid. 



ii. The sphenethmoid or girdle-bone is a bony tube 

 which encircles the anterior end of the cranial 

 cavity, and extends forwards into the olfactory 

 region : in front it is divided by a vertical parti- 

 tion into light and left cavities, in which lie the 

 olfactoiy sacs. 



