24 ANATOMY OF A CHELONIAlSr. 



below it articulates with the exoccipitals and anchy- 

 loses with the epiotic. 



41. The exoccipital bones form the lateral boundaries 

 of the foramen magnum, and, together with the basi- 

 occipital, form the occipital condyle. 



42. The bast-occipital articulates on its sides with the 

 exoccipitals, and anteriorly with the basi-sphenoid. 



43. On the base of the skull, in front of the basi- 

 occipital, is seen the bast-sphenoid. It articulates with 

 the pterygoids in front, and the prootics on its sides ; 

 when removed, observe that its upper or inner surface 

 presents anteriorly a concavity {sella turcica) bounded 

 by ridges, which rise up in front as a pair of thin bony 

 plates, the anterior clinoid processes. Posteriorly its up- 

 per surface presents a median ridge, continuous with 

 a similar one on the corresponding surface of the basi- 

 occipital. 



44. In front of the basi-sphenoid, and partly covering 

 its ventral surface, are the pterygoid bones. 



45. The parietal bones form a large part of the roof 

 of the cranium. Posteriorly they overlap the supra- 

 occipitals ; in front they unite with the frontals ; in 

 the middle line they meet one another ; laterally, they 

 reach down and cover in much of the sides of the brain- 

 case proper, meeting the pterygoids, palate bones, and 

 prootics. 



46. The great downward extension of the parietals 

 is a marked feature of the Chelonian skull, causing 



