ENDOCRANIAL ANATOMY OF OREODON 275 



canal. If a cast be endodural (i. e., a cast of the cavity enclosed 

 by the dural sheath) there can be no trace on it of a sagittal skull 

 groove for the lodgement of the superior sagittal sinus, but in its 

 place will be found a cleft corresponding to the falx and repro- 

 ducing the great sagittal fissure and incidentally the margo med- 

 ialis cerebri. It follows, therefore, that the casts described in 

 this paper and all others formed in like manner by the displace- 

 ment of the soft parts within the skull by mineral debris, are 

 natural endocranial casts. 



Notwithstanding the fidelity of detail with which the cerebral 

 fissural pattern of the lower gyrencephalous mammals is repro- 

 duced upon their endocranial casts, the interpretation of this 

 pattern is frequently a matter of considerable difficulty. This 

 must be in the nature of the case, especially when the secondary 

 sulci are richly developed, since even in the examination of the 

 brain itself in well known modern forms it becomes necessary 

 to open up the fissures in many cases befor6 the certain interpre- 

 tation of their arrangement and homology is possible. 



In concluding these general remarks, comment may be made 

 on the significance of the fact that in all the Anthropoidea and 

 especially in man the convolutions and sulci over the convex sur- 

 face of the cerebrum are poorly reproduced on the endocranial 

 skull surface. In man only on the skull parts abutting upon the 

 orbital and inferior temporal surfaces of the cerebrum, and to a 

 lesser extent on the occipital surface, is the cerebral pattern 

 defined with a sharpness comparable with that obtaining over 

 the whole cerebral endocranial surface in the dog. This, however, 

 is not the case among the Prosmiii. 



The probable explanation of this phenomenon is to be looked 

 for in the comparatively recent origin of the members of the 

 group Anthropoidea, together with the fact that this order as a 

 whole is characterized by the elaboration of certain phylogeneti- 

 cally new neopallial areas which made their appearance in the 

 frontal and temporo-parietal regions and show a gradual increase 

 in their differentiation and expansion as one ascends the anthro- 

 poid scale. The relative length of time during ontogeny in which 

 the cerebrum continues to expand and differentiate is greater 



