ENDOCRANIAL NATURAL CASTS 265 



Measurements in millimeters 



Maximum length of cast 82 



Maximum length of cerebrum 43 



Maximum length of cerebellum 25 



Maximum width of cerebrum 42 



Maximum width of cerebellum 31.5 



Maximum distance between ventral edges of pyriform lobes 16 



Maximum transverse diameter of olfactory bulbs 13 



Maximum vertical diameter of cerebrum with pyriform bodies 31 



Maximum vertical diameter of medulla oblongata and cerebellum 30 



Mesoreodon: There are no brain casts of this genus in the Marsh Collection. Scott (1895, 

 pp. 132-133) described the endocranial cast of M. chelonyx, stating that it is somewhat more 

 advanced and modernized than that of Merycoidodon. He goes on to say: 



This advance is noticeable in the general form of the hemispheres, which, though not broader behind than 

 in some specimens of Oreodon, are much more so anteriorly, and thus the whole brain is fuller, more rounded 

 and tapers less forward. The hemispheres have also slightly increased in vertical diameter, so that they are 

 no longer exceeded in this dimension by the height of the cerebellum and medulla oblongata. Posteriorly, 

 the two halves of the cerebrum are brought closer together and reach the cerebellum in the middle line, not 

 gaping so as to expose part of the optic lobes, as is the case in the White River genus. They do not, however, 

 appear to overlap the lateral lobes any more extensively than in that form. 



The sulci are very nearly the same as occur in some specimens of Oreodon, but they pursue a slightly 

 more sinuous course, which gives an appearance of richer convolutions. The dorsal surface displays ( 1 ) a 

 short and straight lateral fissure, which does not connect anteriorly with the suprasylvian, as is sometimes the 

 case in the White River genus. As regards the latter, Krueg (1878) regards this sulcus as the splenial, which 

 by an extreme degree of "supination" is exposed upon the dorsal surface of the cerebrum, as in many of the 

 small artiodactyls now living. But as this fissure does not extend to the medial surface of the hemisphere, this 

 interpretation does not seem probable. (2) There is a longer and more curved suprasylvian fissure, which is 

 connected anteriorly with the coronal fissure by means of a short and faintly marked ansate sulcus. This 

 connection is also found in . . . Oreodon. (3) The coronal sulcus consists of two portions; the anterior is 

 longer and curves downward and outward, while the posterior is shorter and more obscurely marked and con- 

 verges towards the middle line in a way that suggests the crucial sulcus of the Carnivora. The lateral view 

 shows, in addition to these fissures, a short and nearly horizontal sylvian sulcus and a presylvian which has a 

 more nearly vertical course than in Oreodon. The sylvian fissure appears to be connected with the fissura 

 rhinalis, though in this region the sulci are very obscure and difficult to interpret. Indications of a posterior 

 suprasylvian sulcus are also to be observed. . . . 



In both of the brain-casts of Mesoreodon the olfactory lobes are broken away, but it is plain that they 

 were not at all overlapped by the cerebrum. The cerebellum is very much as in the older White River type; its 

 posterior face rises nearly vertically from the medulla; the vermis is large and prominent and the lateral lobes are 

 broad. 



Promerycochcerus : An endocranial cast of Promerycochoerus superbus, Cat. No. 1 1002 Y.P.M., 

 while of larger size, has very similar proportions to that of Eporeodon. The cerebral sulci (PI. L, 

 figs. 3-4) and gyri are much more shallow ; the transverse fissure is far more open ; and the vermis 

 cerebelli is not nearly so prominent as in the earlier genus. The medulla oblongata is relatively 

 and proportionally much larger. Its vertical diameter is considerably greater than that of the 

 cerebellum, while in Eporeodon the reverse is true. 



The ventral surface exhibits the nerve endings more clearly than in the endocranial cast of 

 Eporeodon. Beginning at the posterior part, the precondylar foramen is well shown, then the 

 foramen lacerum posterius, and just above it laterally are the internal auditory meatus and the 

 eminence representing the fossa of the petrous bone, next the foramen ovale through which passes 

 the mandibular division of the trigeminus nerve, and then, in advance, the foramen lacerum anterius 

 with the ophthalmic and maxillary divisions of this same nerve (nerve V). The chiasma opticum 



