304 DAVIDSON BLACK 



(Holl) occupies a very rostral position. The rostral opercular 

 fold, over the ramus anterior ectosylvii, is not greatly developed 

 in any of the specimens of the series. The rostral and caudal 

 opercula meet to form a long processus acuminis ectosylvii which 

 almost reaches the suprasylvian sulcus. In the configuration of 

 the ectosylvian complex, Oreodon thus reveals certain unique 

 as well as suilline characters. 



A notch-like sulcus arises from the rhinal fissure on a plane 

 with the foramen lacerum anterius, which corresponds to the 

 level at which begins the large caudal expansion of the pyriform 

 lobe. This notch, being the result of cortical tension due to the 

 downfolding of the neopallium behind the region of its fixation, 

 probably represents the approximate caudal limit of the under- 

 lying corpus striatum as in modern ungulates (v. Holl, 1. c). It 

 is therefore to be considered as the true homologue of the pseudo- 

 sylvian sulcus of the modern carnivores and ungulates (cf. Holl, 

 also Elliot Smith, 10). 



In the area caudal of the processus acuminis ectosylvii and 

 parallel to it, there is found a sulcus which has been termed the 

 ramus descendens suprasylvii because of its resemblance to the 

 sulcus bearing this name in Sus. Holl (1. c.) has considered this 

 sulcus to be the oblique, but as a rule in Sus both the sulci men- 

 tioned are present as independent furrows. In any case it would 

 seem that the sulcus in question has made its appearance in both 

 Oreodon and Sus to fulfill similar requirements, viz., to relieve 

 the tension consequent on the sagittal expansion of the caudal 

 ectosylvian operculum. 



The rostral boimdary of the trigonum Sylvii is marked by a 

 groove which must correspond to the presylvian sulcus of modern 

 ungulates. Apparently it is continuous caudally with the ramus 

 anterior ectosylvii as in Hydrochoerus (Holl, 1. c.) and as fre- 

 quently occurs in Sus. Frontally it soon becomes hidden 

 beneath the overhanging rostrolateral cortical prominence but it 

 is possible that it may have been prolonged around the frontal 

 pole of the cerebrum to become continuous with a small notch 

 on the mesial surface of the latter. In any case the area of cor- 

 tex rostral to the presylvian sulcus in Oreodon must have been 



