ENDOCRANIAL NATURAL CASTS 



267 



The gyn of the Eporeodon cerebrum are neither particularly deep nor numerous. In general 

 terms, the brain cast of that genus indicates that its reflex centers were well developed, basing this 

 statement on the large relative size of the cerebellum. The large olfactory centers indicate a very 

 acute sense of smell, as in the carnivores. The optic nerves were large, thus postulating good vision. 

 The cerebrum was apparently not of high importance. In other words, it would seem that the life 

 habits of this family were based on a complex of reflexes, with very little volitional activity, result- 

 ing in creatures mainly of habit and routine. This in itself was possibly one of the fundamental 

 causes of the extinction of the group. 



In Promerycochcerus (John Day phase) there seems to have been an actual retrogression in that 

 the gyri and sulci are much less pronounced than in either Merycoidodon or in Eporeodon. That 

 is, the cerebrum of this genus is smoother than in the earlier forms, and yet its actual surface and 

 volume are proportionally no greater. We should expect the opposite to be the case, if the later 

 forms had advanced in mental equipment beyond the ancestral groups. 



Scott (1895, p. 134) stated that the character of the cerebral sulci of Mesoreodon is very much 

 like that of the smaller and more primitive recent ruminants. These Krueg (1878) has shown to 

 agree very closely in fundamental plan with the carnivores. 



The convolutions of the cerebrum of Cyclopidius, one of the most specialized of the whole 

 group, are poorly defined, showing retrogression in this genus, as in Promerycochcerus. 



From this study of the endocranial casts of these several genera of merycoidodonts, it seems 

 to me that the facts to be emphasized are: first, that the proportions of the various brain elements 

 remained very nearly constant; second, that there was no relative increase in the cerebrum in the 

 later forms and therefore apparently no great increase in volition or intelligence; third, that 

 Promerycochcerus and Cyclopidius show actual retrogression in that the cerebral surfaces were rela- 

 tively smoother and in the former the medulla oblongata is larger than in the others; and, fourth, 

 that there seems to be conclusive evidence that this group lacked the mentality, especially in the later 

 forms, to discover ways and means of adapting its members to the changing geologic conditions which 

 took place in the Miocene and Pliocene. 



Black (1920A, pp. 311-312) has summarized the characters shown in the brain cast of Mery- 

 coidodon, and since all of the genera show no marked advance in general features over this Oligo- 

 ceneform, his conclusions apply to the entire family, so far as we know now. I shall quote a part 

 of his findings, as follows: 



A, frimitwe characters: small volume of its cerebrum in comparison with allied modern forms of similar 

 bodily dimensions; limited caudal expansion of the neopallium as evidenced by the exposed dorsum cerebelli; 

 practical absence of presylvian neopallial area; apparently uninterrupted intracranial course of the internal 

 carotid artery (absence of rete mirabile). B, characters of rhmencephalon: large terminal olfactory bulbs and 

 extensive lamina cribrosa; massive olfactory tractus and bulbus; macroscopic tractus bulbo-tuberculare ; pyriform 

 lobes well developed and very large in proportion to the bulk of the cerebrum above the rhinal fissure. C, 

 artiodactyl characters: foramen lacerum anterius transmitting ophthalmic and maxillary division of the trigeminus; 

 foramen ovale for mandibular division of trigeminus; long processus acuminis ectosylvii at junction of rostral 

 and caudal ectosylvian opercula; pseudosylvian sulcus as in modern carnivores and ungulates; lateral sulcus in 

 no case communicating rostrally with corono-ansate ; presence of small ramus lateralis suprasylvii of Holl; 

 insular cortex defined by anterior and posterior ectosylvian sulci, rhinal fissure and presylvian sulcus; sigmoid 

 curvature of lobulus medianus posterior of the cerebellum; loop formation of formatio vermicularis cerebelli. 

 In the final analysis therefore it is evident that Orcodon was a primitive macrosmatic artiodactyl. 



Oreodon presents, however, other and more determinate endocranial characters which, while confirming 

 its artiodactyl rank, render its status within that order difficult to define. These characters fall naturally into 

 two categories as follows: A, ruminant characters: junction of the corono-ansate complex with the suprasylvian 

 sulcus in every hemisphere but one examined; relations and extent of lobus anterior and lobulus simplex cerebelli; 

 presence of parieto-temporal canal. B, sudline characters: internal carotid artery transmitted through the 

 posterior lacerated foramen; caudal ectosylvian operculum extensive; ramus descendens suprasylvii and ramus 



