88 



BULLETIN OF THE 



gests the crucial sulcus of the Carnivora. The sylvian fissure is very shallow. 

 The temporo-sphenoidal lobe is very prominent, and is divided into three gyri 

 (s, m, i) by two sulci. Beneath the third frontal gyrus is a vertical sulcus, par- 

 allel with the sylvian. 



The cerebellum has a large central lobe with transverse sample furrows. 



The Dentition. 



There are a few new points to be noted in regard to the teeth of Mesohippus, 

 which bear upon the dentition of the horses in general, and are clearly shown in 



"v* p n 



JVOU 



Figure 11. — Superior and inferior molars of Mesohippus Bairdii X ^« 



a series of unworn crowns of the upper and lower jaws. Scott has already 

 pointed out that the incisors in this genus are simple, there being no indication 



of the infolding of the enamel, such as is seen in 

 Anchitherium aurelianense. In some of the John 

 Day species of Anchitherium the enamel is not in- 

 folded, as observed in the lower jaw of a specimen 

 referred to A. equiceps, Cope. 



The upper molars of Mesohippus clearly show the 

 first step in the formation of the posterior pillar, pp, 

 which is so conspicuous a feature in Anchitherium, 

 in the posterior valley. This can also be observed 

 in a still simpler stage in a specimen of Anchilophus 

 from the French Phosphorites. Step by step with 

 the development of this cusp appears the posterior 

 pillar, p, in the lower molars, behind the entoconid ; 

 this accessory cusp can be traced back to the teeth 

 of Epihippus. When it finally unites with the en- 

 toconid, in Hipparion, it forms the posterior twin 

 cusp (b, b, Riitimeyer), which is analogous to the 

 anterior pair formed by the union of the metaconid 

 and anterior pillar, a (a, a, Riitimeyer). 

 Thus the transition from the Mesohippus to the Anchitherium molars is very 

 gradual, as shown in the accompanying figures. By tracing back the rise of 



/vet, 



TTV 



me 



Figure 12. — Superior mo- 

 lar of Anchitherium lonyi- 

 critte X y. Superior and 

 external view. Cope col- 

 lection. 



