DEVELOPMENT OF THE SKULL OF EMYS 723 



up of two parallel plates which are united to each other only 

 along the ventral margin by a high coniniissure (figs. 22 and 23), 

 a condition which apparent!}' may be referred to a primitive fonxi 

 in which the skull was platybasic. .Interiorly the septmn inter- 

 orbitale passes without interruption into the septum nasi, which 

 in my youngest stages is apparently of unpaired origin. 



The temporal region is made up of the trabeculae and pilae 

 prooticae, both arising from the anterior aspect of the crista 

 seUaris, the latter from the dorsal side, the former from the ven- 

 tral side. The bases of these two structures are separated from 

 each other by the nerviis abducens which, as already mentioned, 

 penetrates the crista sellaris ventral to the base of the pila prootica 

 (VI. fig. 8). 



The trabeculae project horizontally forward in the plane of 

 the basal plate as triangular rods which converge in the median 

 line and enclose the semicircular fenestra hypophyseos in front 

 and on the two sides. They continue further forward as the 

 tliickened ventral margin of the septum interorbitale and fuse so 

 that no trace of the paired nature of the septum remains. Pos- 

 teriorly the trabeculae are continuous -n-ith the antero-lateral 

 margins of the basal plate which represent the crista basiptery- 

 goidea, as already described. 



At the level of the posterior margin of the fenestra hypophyseos 

 a separate cartilago articularis, described by Gaupp in Lacerta, 

 is represented by a rudunentarj^, imperfect^ chondrified mass of 

 tissue which is attached to the ventral edge of the crista basi- 

 pterygoidea (c.a., fig. 8). This cartilage forms a small, roundish 

 knob, about two-thirds as broad as long, which projects down- 

 ward from the crest with its free end directed anteriorly and ven- 

 trally. It extends lateral to the ramus palatums n. faciahs and 

 its free end is embraced laterallj- as well as ventrally bj^ the ptery- 

 goideum. In older embrj'os the crista basipterygoidea becomes 

 relativel}^ more prominent and is partly smTOunded by the dorso- 

 median portion of the pterygoideum, and the cartilago articu- 

 laris disappears as a distinct piece, probabh' fusing ^ath the 

 crest (c.pt., fig. 20). 



