ENDOCRANIAL NATURAL CASTS 261 



Oreodon. In contrast to this primitive condition of neopallial development, the cerebellum in Oreodon, in 

 the arrangement and development of its parts, shows a plan of organization very similar and in no way inferior 

 to that obtaining in modern artiodactyls, while its bulk compares quite favorably with the volume of this organ 

 in Sus. 



Although but little doubt can remain as to the homology with the fissura prima of the well marked 

 transverse depression visible on the dorsum of these casts, the anterior lobe and lobulus simplex will be con- 

 sidered together since it is the combined features of these parts that reveal their essentially ruminant characteristics. 



The area in question is clearly marked out ... as that part of the cerebellum which lies directly rostrad 

 of the prominent postero-median lobule and whose division into medial and lateral moities is not sharply indicated 

 by grooves. The folia in the mid-dorsal region of this area have expanded to form a characteristic elongated 

 median prominence which merges gradually into the depressed, laterally situated portions. In other words, the 

 chief growth expansion of the folia in this region has been in the sagittal direction and has been greatest in the 

 mid-sagittal plane. . . . 



Laterally the depressed portions of this region are encroached upon by the forward growth of the ansiform 

 lobules and so become much reduced in rostro-caudal extent. In spite of this, the part in question is in contact 

 laterally with the crus ascendens of the formatio vermicularis for a considerable distance, being separated there- 

 from by the rostral part of the fissura parafloccularis. 



In all the above features the cerebellum of Oreodon shows marked resemblance to that of ruminants 

 (e.g., Ovis, Bos, Cervus) and presents an equally marked contrast to that of Sus. 



Lobulus medianus posterior. In Oreodon this division of the posterior lobe is strongly developed and is 

 folded upon itself to form a sigmoid-shaped lobule which protrudes high above the level of the adjacent anso- 

 paramedian area. A similar development of this lobule is characteristic of most modern ungulates, the few 

 exceptions being chiefly among the smaller members of the group (e.g., Hydropotes). 



Lobulus ansiformis. This lobule is relatively well developed in Oreodon and its folia are apparently arranged 

 in the form of two crura, separated by a short intercrural sulcus. Among modern ungulates a loop-like ansiform 

 lobule is characteristically developed in Sus in which the lobule is essentially similar to the corresponding region 

 in carnivores. In Bos and Equus a somewhat similar condition obtains, though in these animals the loop- 

 formation is much less extensive. In other ungulates, however, the two crura of the ansiform lobule and the 

 intercrural sulcus can rarely be distinguished. 



Bolk has pointed out that the ansiform lobule constitutes an unpaired center for the elaboration of tonic, 

 sthenic and static impulses for the musculature of the homolateral limbs and in consequence is most highly 

 developed in those forms in which the power of independent limb action has been most perfectly acquired. 

 For this reason the lobule in question is more typically developed in carnivores than in ungulates. 



In the latter group, the functional reduction of the limb elements has progressed to the smallest extent 

 in suillines; and in Sus, where both forearm and leg elements are retained as complete and distinct bones, the 

 ansiform lobule is relatively large and characteristically loop-shaped. Among the ungulates, though exceptions 

 occur as noted above, it is evident that a correlation does exist between the development of this lobule and the 

 functional development of the limbs. On this account it is of interest to recall that in Oreodon the bones 

 of the forearm and leg were complete and separate and further that it is probable that a clavicle was also present. 



With regard, therefore, to the characters of its ansiform lobule, Oreodon apparently presents resemblances 

 to both suillines and certain ruminants. 



Lobulus faramedianus. In Oreodon this subdivision of the posterior cerebellar lobe is overlapped dorso- 

 lateral^ by the ansiform lobule so that it comes to lie at the bottom of a depressed area between the latter 

 lobule and the lobulus medianus posterior. 



Formatio vermicularis. This region of the cerebellum is well developed in Oreodon and is arranged on 

 a plan essentially similar to that obtaining among ruminants (e.g., Ovis, Cervus). At the caudal end of the 

 pars tonsillaris the ascending crus is apparently in continuity with the paramedian lobule while rostrally this 

 crus abuts upon the lateral border of the lobulus simplex and lobus anterior. Here it turns upon itself to 

 become continuous with the crus descendens from which it is separated by the intratonsillar sulcus. The 

 descending crus is shorter than the ascending limb and terminates at the level of the internal acoustic meatus. 



At its caudal termination the folia of the crus descendens form a short non-pedunculated projection which 

 is lodged in the well developed subarcuate fossa of the petrous bone. This projection therefore constitutes 

 a homologue of the lobulus petrosus as defined by Bolk in carnivores, but is not homologous with the cerebellar 

 parts lodged in the socalled "floccular fossa" of modern ungulates. . . . 



