ENDOCRANIAL ANATOMY OF OREODON 305 



very small indeed. Thus, while a most primitive arrangement 

 of the part obtains, the presylvian region in Oreodon presents 

 certain suilline resemblances. 



Cortical projection areas. The general plan of cortical locali- 

 zation has probably been essentially similar in all mammals 

 since the first establishment of neopallial projection centers (v. 

 Elliot Smith, IJf). The truth of this conception being granted, 

 the facts as they emerge in the foregoing discussion suggest the 

 following tentative deductions: 



1) The visual projection field probably occupied the cortex 

 medial to the lateral sulcus (v. Brodmann, 5). 



The gyrus in question is in all cases broad and well developed, 

 and since secondary folding has not been observed it is presumed 

 that it was established early in ontogeny and did not require to 

 alter its original simple configuration in order to attain the 

 requisite area. 



2) The auditory projection and association fields probably 

 occupied the lateral neopallium caudal to the processus acuminis 

 ectosylvii, both on the deep and exposed surface of the operculum 

 (v. Campbell, 6). 



The area in question is well developed and its chief direction 

 of expansion has evidently been in the sagittal plane, since not 

 only has the posterior operculum been fully developed but its 

 lateral surface has been further increased in area by the develop- 

 ment of the ramus descendens suprasylvii. Since both the lat- 

 ter sulcus and the processus acuminis are of simple linear nature, 

 it is presumed that they were not subject to the action of trans- 

 versely directed cortical stresses during their development and 

 before they became established in much their present form. It 

 is presumed, therefore, that this area was laid down compara- 

 tively early in ontogeny (at or about the same time as the visual 

 field) and before the expansion of the prominent gyrus along its 

 medial boundary. The latter gyrus is to be considered as an 

 association field developed later in ontogeny and owing its promi- 

 nence to its inability to expand laterally. 



The topography of such fields as those mentioned above was 

 no doubt the chief factor underlying the great lateral expansion 



