728 B. W. KUNKEL 



septale. The orbital portion of the obito-temporal region is 

 characterized by the septum interorbitale which increases in 

 height from behind and carries the planum supraseptale with its 

 dorsal margin so that the floor of the cranium is raised. 



The septum interorbitale is a triangular plate lying in the sagit- 

 tal plane, arising posteriorly from in front of the fenestra hypo- 

 physeos and rai:)idly increasing in height anteriorly almost to 

 its extreme anterior end where the dorsal margin inclines rapidly 

 ventrally to pass into the septum nasi. The ventral margin is 

 thickened throughout its length to a cylindrical rod which is 

 continuous behind with the trabeculae and tapers anteriorly in 

 the region of the olfactory capsule to a thin edge. Its dorsal 

 margin is continuous with the subiculum infundibuli and planum 

 supraseptale except in the region of the fenestra optica where 

 it is free. The septum forms a continuous plate throughout with 

 no fenestrae such as Lacerta exhibits, although at a later stage, 

 as already mentioned, a long slit in the posterior portion of the 

 septum ventral to the fenestra optica causes a cartilago hypo- 

 chiasmatica to be differentiated. 



Of especial interest in the stage modelled is the structure of 

 the extreme anterior dorsal corner of the septum where it passes 

 over into the planum supraseptale posteriorly and the commissurae 

 spheno-ethmoidales anteriorly. At this point the septum is very 

 clearly paired, the right and left halves being separated from each 

 other by a shallow groove which is continuous postero-dorsally 

 with the troughlike planum supraseptale. By a comparison 

 with younger stages the significance of this peculiarity becomes 

 clear. In an embryo having a carapace length of 7 mm. the 

 portion of the septum lying in front of the fenestra optica is 

 represented by two vertical plates, parallel to each other which 

 are fused together along their ventral edges (fig. 23). Dorsally 

 these plates diverge rapidly from each other along a line corre- 

 sponding approximately to the dorsal margin of the septum in the 

 older embryo. In the region of the fenestra optica, and for a 

 short distance in front of the fenestra, the septum is thick so that 

 its ventral cylindrical margin is not differentiated from the rest 



