310 SCOTT. [Vol. V. 



alisphenoid canal. The large space surrounding the small tym- 

 panic bone includes the Eustachian canal, foramen lacerum 

 medium and posterius, and the carotid canal ; in none of the 

 fossils which I have seen can these foramina be distinguished. 



The numerous venous foramina which perforate the parietal, 

 are in the horse shifted to the squamosal. In the mandible there 

 are three small mental foramina, extending from pin. 3 to the 

 diastema. 



When viewed in longitudinal section, the skull of Mesohippus 

 (PI. XXIIL, Fig. 17) presents many important differences from 

 that of Equus. Especially striking is the small size of the nasal 

 chamber and the large size of the brain cavity, especially in the 

 antero-posterior direction. The cerebellar fossa is decidedly 

 larger in proportion, and is less overlapped by the cerebral fossa. 

 There is very little thickening or diploetic structure in the pos- 

 terior parietal and supra-occipital regions, and the alisphenoids 

 and orbito-sphenoids are excessively thin. The frontal sinus is 

 exceedingly small, and does not extend over the hemispheres. 

 The nasal chamber is low and narrow, and the maxillary sinus 

 much smaller than in Equus. Unfortunately the specimen gives 

 us no information as to the condition of the vomer or of the 

 maxillo- and ethmoturbinals. 



I have not been able to see a well-preserved skull of the 

 European Anchitherium, nor, so far as I am aware, has any such 

 specimen been figured. Kowalevsky's drawings (No. 25, PI. III., 

 Figs. 50-52, 56, and 57) show some differences from Mesohippus, 

 especially in the increased thickness and width of the alveolar 

 portion of the premaxillaries and of the symphysis mandibuli, 

 corresponding to the development of the incisor teeth. Anchi- 

 therium appears to agree with Mesohippus in having a well- 

 marked pre-orbital depression. 



In the skull of Equus the principal changes are as follows : 

 (1) The face has become greatly elongated and increased both 

 in vertical and transverse diameters. (2) The alveolar portion 

 of the maxillaries and premaxillaries has become very much 

 higher, in correspondence with the development of prismatic 

 teeth. (3) The anterior nares are much higher and wider, the 

 nasals longer and tapering anteriorly to sharp points. (4) There 

 is no pre-orbital depression. (5) The jugal and lachrymal are 

 much more extended upon the face, and the masseter crest 



