ENDOCRANIAL NATURAL CASTS 



Natural brain casts, representing several genera of the merycoidodonts, are found, though 

 rarely, in an excellent state of preservation. Since the brain is one of the most conservative com- 

 ponents of any organism and is far less affected by changes of environment than are the teeth and 

 feet, it is consequently of great value in affording clues to relationships and to the evolutionary 

 history of the various groups of fossil forms. We shall examine in some detail the known casts of 

 the different genera represented. 



Protoreodon: The most ancient, of which we have knowledge, is the brain cast of Protoreodon, 

 which Scott (1899, pp. 92-93) described as follows: 



The brain is relatively smaller than in Oreodon and much more simply convoluted. The hemispheres are 

 particularly small, leaving the cerebellum entirely uncovered and, apparently, even a portion of the corpora quad- 

 rigemina. The cerebrum is pear-shaped and narrows anteriorly, abruptly so in front of the temporo-sphenoidal 

 lobes; the latter are proportionately quite large. The convolutions are very simple and take a longitudinal course. 

 On the dorsal surface of the hemisphere only two sulci are visible, one of which is the lateral and the other may 

 be the suprasylvian, though they come together in front, enclosing a pyriform gyrus between them. The shape of 

 the hemispheres is quite as much like those of Leptomeryx as it is like those of Oreodon. The posterior region of 

 the brain, including the cerebellum and medulla, is very long proportionately. 



Merycoidodon: Black (1920A) gave so complete a description of several endocranial casts of 

 Merycoidodon culbertsonii that I shall use excerpts from his discussion. 



The large olfactory bulbs are set obliquely upon the rostral extremity of the massive olfactory 

 tracts. It is probable that the lamina cribrosa extended well back over the dorsal surface of the 

 bulbs, which in their shape and relative size closely resemble those of Orycterofus. 



Black wrote that: 



As in Orycterofus, the large olfactory peduncles are visible in a dorsal view of the brain. Ventrally they give 

 rise to the medial and lateral olfactory tracts. From the latter, a series of fibers arise which pass obliquely over the 

 rostro-lateral margin of the prominent olfactory tubercles and constitute the tractus bulbo-tuberculare. 



Caudal of the tuberculum and between the optic chiasma and the pyriform lobe lies the locus perforatus 

 amicus. ... At the rostro-mesial angle of the ventral surface of the broad caudal expansion of the pyriform 

 lobe, a small circumscribed eminence is present, . . . evidently corresponding to the "gyrus lunaris" of Retzius 

 which, as Elliot Smith has shown, is really the surface of the nucleus amygdala;. The pyriform lobes are very 

 large in proportion to the size of the neopallium and on either side are sharply demarcated therefrom by the well 

 defined rhinal fissure. In short it may be said that the rhinencephalon in Oreodon is developed in a manner 

 characteristic of a highly macrosmatic mammal. . . . 



(The sulcus lateralis) is well marked and shows no tendency towards duplication, branching or other irregu- 

 larity. The sulcus . . . forms a sharply cut groove parallel to the median border of the hemisphere and 

 separated therefrom by a broad slightly depressed gyrus. Caudally it does not extend below the level of the con- 

 fluens sinuum and rostrally it reaches to within a short distance of the corono-ansate sulcus, but in no instance 

 joins the latter. In the relations and morphology of its lateral sulcus, Oreodon thus conforms to a generalized 

 and primitive ungulate type. 



The coronal sulcus forms a deep, well marked furrow which reaches almost to the frontal pole of the 

 cerebrum and in every specimen the gyrus on its medial side is high and prominent. Caudally the coronal is 

 continuous with an obliquely placed ansate sulcus whose mesial end is closely associated with the margin of 

 the hemisphere and probably cut the latter, as frequently happens among modern artiodactyls (e.g., Ovls and Bos). 



The corono-ansate complex may be independent of the suprasylvian, but is usually joined to the latter 

 sulcus .... The junction of the coronal and suprasylvian sulci in this manner is a condition which character- 

 istically obtains in many modern ruminants. Among suillines, on the other hand, such a condition but rarely 



