260 THE MERYCOIDODONTID/E 



occurs and the coronal sulcus is usually joined with the upturned intercalary, a feature which serves to distinguish 

 the members of this group from other artiodactyls. It emerges, therefore, that the relations of the corono-ansate 

 complex in Oreodon are of a predominantly ruminant character. 



The suprasylvian sulcus extends from the caudal pole of the hemisphere to the level of the ansate sulcus 

 in the form of a wide arch. Its caudal extremity shows a tendency towards bifurcation and the development 

 of a short caudally directed limb. . . . 



Rostral of the level of the processus acuminis ectosylvii, a small laterally directed notch is frequently 

 present on the suprasylvian sulcus which may correspond to the ramus anterior suprasylvia. This sulcus has 

 been termed by Holl the ramus lateralis suprasylvia. It may be present in varying degree in any member 

 of the ungulate group .... The gyrus situated between the suprasylvian and lateral sulci is especially 

 prominent .... 



Ventral to the suprasylvian arch the cortex is folded to form two opercula in a characteristic artiodactyl 

 fashion. ... the caudal operculum which forms the enlarged caudal lip of the ramus posterior ectosylvii is 

 as highly developed as in Sits, and as a consequence the trigonum Sylvii (Holl) occupies a very rostral position. 

 The rostral opercular fold, over the ramus anterior ectosylvii, is not greatly developed .... 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 underlying corpus striatum as in modern ungulates. It is there- 

 fore to be considered as the true homologue of the pseudosylvian sulcus of the modern carnivores and ungulates. 



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 has considered this sulcus to be the oblique, but as a rule both the sulci mentioned 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 boundary of the trigonum Sylvii is marked by a groove which must correspond to the presyl- 

 vian sulcus of modern ungulates. Apparently it is continuous caudally with the ramus anterior ectosylvii as 

 in Hydrochcerus and as frequently 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 cortex rostral to the presylvian sulcus in Oreodon must have been very small indeed. Thus, while a 

 most primitive arrangement of the part obtains, the presylvian region in Oreodon presents certain suilline 



resemblances. 



The general plan of cortical localization has probably been essentially similar in all mammals since the 

 first establishment of neopallial projection centers. 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. . . . 



(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. . . . 



The topography of such fields as those mentioned above was no doubt the chief factor underlying the 

 great lateral expansion of the neopallium in Oreodon, the transverse diameter of whose cerebrum practically 

 equalled that of Sus. 



(3) The general somatic sensory projection area possibly lay lateral to the corono-ansate sulcus and extended 

 caudally as far as the processus acuminis ectosylvii. 



(4) The motor projection area probably occupied the cortex medial and to a small extent lateral to the 

 corono-ansate sulcus and extended rostrad to the presylvian sulcus. . . . 



The dorsal surface of the cerebellum in Oreodon was fully exposed and even a small area of the midbrain 

 must have been visible from above, owing to the slight caudal development of the occipital portion of the 

 neopallium. Among modern forms the nearest approach to such a primitive cerebral condition is to be seen among 

 the Edentates, though in all of the latter forms the occipital neopallial expansion is more extensive than in 



