SKULL — CHONDROCRANIUM 



63 



and some other Ichthyopsida it has a more or less complete roof 

 (tegmen cranii) formed by the synotic tectum behind, farther 

 rorwards by growth from the side walls of the interorbital and nasal 

 fegions, all meeting in the medial line dorsal to the brain. The 

 floor is completed between the eyes by growths from the trabeculae 

 across the hypophysial fenestra, completely separating the hypophy- 

 sis from the roof of the' mouth. In most Vertebrates hypophysis 



Fig. 67. — Schema of tropibasic cranium (Gaupp, '04). ap, antorbital plate; 

 hp, basal plate (parachordals) ; c, carotid foramen; col, columella; /a, /e, foramina apicale 

 and epiphaniale; fh, fenestra hypophyseos; In, mn, lateral and medial nasal nerves; 

 nc, nasal capsule; oc, otic capsule; so, foramina for spino-occipital nerves; /, trabecula; 

 Ic, trabecula communis; ii-x, nerve exits. 



and infundibulum from this time occupy a pit (hypophysial fossa) 

 In the floor of the cranium, fossa and its anterior and posterior borders 

 constituting the sella turcica (Turkish saddle), an important land- 

 mark in the skull. 



The primitive cranium just described undergoes modifications in 

 the higher classes, some of which are described in connexion with the 

 separate groups, but a few general points are summarized here. 

 The primitive chondrocranium is platybasic (fig. 131), having a large 

 and wide hypophysial fenestra, and is found only in Ichthyopsida, 

 and there only in species with a depressed head and relatively small 



