BONY SKULL 



575 



ossify to form the paired orbitosphenoids, which make the orbital 

 septum when this is present, and otherwise make much of the 

 wall of the orbit. The orbitosphenoid may generally be dis- 

 tinguished from other bones near it by the fact that it is pierced 

 by the large optic foramen for the optic nerve. In marsupials, 

 however, this has moved back so that it is confluent with the 

 foramen lacerum anterius, and merely makes a notch in the 

 posterior border of the bone. In many mammals the two orbito- 

 sphenoids meet and fuse ventrally and so form part of the floor 

 of the skull. When they do this their ventral portion is called 

 the presphenoid, but it is not strictly a separate bone. The main 



Fig. 448. — The skull of a cod, from the left side.— From Reynolds. 



I and 2, Lacrimal ; 3, median ethmoid ; 4, frontal ; 5, supraoccipital ; 6, pterotic ; 7, orbit ; immediately 

 below is the parasphenoid ; 8, pterygoid; 9, suborbital; 10, bones of orbital ring; 11, maxilla; 

 12, premaxilla ; 13, hyomandibular ; 14, symplectic ; 15, quadrate ; 16, metapterygoid ; 17, opercular; 

 18, subopercular ; 19, preopercular ; 20, interopercular ; 21, articular ; 22, dentary. 



part of the floor of the skull, behind the presphenoid when it is 

 present, and in the region of the trabeculae and basal plate, is 

 formed from a single bone, the basisphenoid. It either retains a 

 hypophyseal fenestra, as in reptiles and birds, or bears on its 

 upper surface a hollow for the pituitary ; in mammals it has a 

 somewhat elaborate cave, the sella turcica, excavated in its upper 

 surface. The bone which develops in the side wall above the 

 basisphenoid on each side is generally termed ' alisphenoid ', 

 but the bones in the different classes to which this name is apphcd 

 are quite certainly not all homologous. That of fishes is best 

 called a pterosphenoid, that of reptiles and birds a pleurosphenoid. 

 The ' ahsphenoid ' of mammals is not an ossification in the 

 chondrocranium, and will be described in its place below. 



