DEVELOPMENT OF THE SKULL OF EMYS 729 



of the septum. In this young stage, accordingly, the cavum 

 cranii extends ventrally between the two plates of the septum 

 in the region between the eyes as a deep, narrow cleft. 



The planum supraseptale forms a broad shallow trough in 

 which the cerebral hemispheres and olfactory lobes rest. Its 

 plane rises obliquely from behind, on account of the increasing 

 height of the septum interorbitale, but at its extreme anterior 

 end its plane falls rapidly ventrally so that it is almost vertical, 

 a condition associated with the rotation ventralwards of the olfac- 

 tory capsule as will be described later under the ethmoidal region. 

 In general, it is broadly oval in form, somewhat wider than long, 

 with its anterior and lateral margins evenly rounded, but its pos- 

 terior margin irregularly indented as has been described above. 

 Medially its posterior margin forms the front boundary of the 

 fenestras opticae and further laterally the front boundary of the 

 fenestra metoptica; between these two fenestrae is the pila me- 

 toptica. The postero-lateral angle of the planum supraseptale is 

 produced behind as a slender rod-hke process which approaches 

 the front edge of the pila prootica. The postero-lateral portion 

 of the planum supraseptale, corresponding somewhat in position 

 to that of the fenestra epioptica of Lacerta, is perforated by 

 numerous small foramina which represent the first stages in the 

 absorption of the cartilage in this part of the cranium. 



At its anterior end, the planum supraseptale becomes very 

 narrow and passes into the anterior end of the septum inter- 

 orbitale. At the same time the two halves of the planum supra- 

 septale, which fonn an obtuse angle with each other throughout 

 most of their length, come to lie at the extreme anterior portion 

 with their median surfaces almost parallel and separated from 

 each other by a narrow cleft. 



The commissiu-ae spheno-ethmoidales, diverging from each 

 other, extend in an antero-ventral direction, from the extreme 

 anterior margin of the planum supraseptale and serve to connect 

 the walls of the olfactory capsule with the orbital region. They 

 enclose the fenestrae olfactoriae laterally. 



