DEVELOPMENT OF THE SKULL OP EMYS 723 



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

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

 a condition which apparently may be referred to a primitive form 

 in which the skull was platybasic. Anteriorly the septum 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 

 sellaris, 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 nervus abducens w^hich, as already mentioned, 

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

 (F/, 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 

 thickened 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 with 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 rudimentary, imperfectly 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 palatinus n. faciahs and 

 its free end is embraced laterally as well as ventrally by the ptery- 

 goideum. In older embryos the crista basipterygoidea becomes 

 relatively more prominent and is partly surrounded by the dorso- 

 median portion of the pterygoideum, and the cartilago articu- 

 laris disappears as a distinct piece, probably fusing with the 

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



