No. 3. ] OSTEOLOGY OF MESOHIPPUS AND LEPTOMER YX. 3 1 1 



has become very prominent, extending far forward upon the 

 maxillary. (6) The maxillary alveolus extends farther behind 

 the last molar. (7) The posterior nares are placed far back, and 

 the posterior part of the palatines is not in contact with the 

 alveoli. (8) The orbits are completely enclosed, much elevated in 

 position, and pushed back of the molar series. This shifting 

 causes important changes in the shape of the alisphenoids. 

 (9) The zygomatic arch is shorter and more massive. (10) The 

 cranial cavity is broader and more rounded, and the cerebral fossa 

 overlaps the cerebellar. (11) The tympanic is ankylosed with 

 the periotic. (12) Large sinuses are developed in the frontals 

 and diploetic thickenings in the median line of the parietals And 

 occipitals. (13) In the lower jaw the symphysis is broadened, 

 procumbent, and in the same line with the rest of the horizontal 

 ramus ; the ascending ramus is higher and narrower, the coronoid 

 straighter and less pointed, and the condyle higher above the 

 level of the molars. 



III. The Brain. 



The brain of Mesohippus has been described and figured by 

 Osborn (No. 42, p. 87), who says of it : "Mesohippus had a large 

 and well-convoluted brain. The length and breadth indicate 

 that it weighed about one-third as much as the brain of the 

 recent horse, while if we estimate the body weights of the fossil 

 and recent animals by the relative size of the humeri, the brain 

 of the Miocene species was proportionally heavier. The cere- 

 brum of the horse is, however, much more highly convoluted, and 

 the frontal lobes are relatively broader. The Mesohippus brain is 

 distinguished in a marked manner by the longitudinal direction of 

 the parietal and occipital sulci, and by the deep transverse frontal 

 sulci, from the oblique sulci of all recent ungulates. In fact, in 

 this respect it bears a marked general resemblance to the brain 

 type of recent Carnivora, and conforms with the higher Ungulata 

 of the Eocene." To this may be added that the hemispheres 

 are narrower and less capacious in the fossil, and as in all the 

 lower members of the ungulate series, they taper much anteriorly. 

 This brain shows in the parietal and occipital region very close 

 agreement with the principal fissures of the equine brain as fig- 

 ured by Krueg (No. 27, Taf. XXIII.), but in the frontal region 

 the agreement is much less close, owing to the transverse direc- 

 tion of the frontal sulci. 



