304 SCOTT. [Vol. V. 



molar shows a beginning of the peculiar shape seen in the horse, 

 not being quadrate in outline, but narrower in front than behind, 

 and having a forward extension of the antero-external angle. 



Although the premolars have thus entirely assumed the molar 

 pattern, yet the development of them as presented by the Eocene 

 genera, shows that, just as in the Artiodactyls, the functionally 

 similar parts of the crown are not homologous in the two 

 series. 



The molars of Anchitherium differ very slightly from those of 

 Mesohippus, the median ribs of the external lobes are less dis- 

 tinctly shown, the transverse crests, which in the American form 

 do not quite reach the outer wall of the crown, are connate with 

 the ectoloph ; the median cusps, para- and metaconules, are rela- 

 tively smaller, and the posterior pillar larger. In both genera the 

 teeth are typically brachyodont. 



2. Lower Jaw. The incisors of Mesohippus are small, with 

 narrow crowns, and simple, sharp cutting edges, which are 

 abruptly truncated. The canine is also small and erect. The 

 first premolar is a minute compressed conical tooth, implanted by 

 a single fang, and not separated by a diastema from the second. 

 The second premolar is not quite molariform, the posterior cres- 

 cent only being developed, the anterior portion of the tooth 

 being compressed and somewhat trenchant. The third and 

 fourth premolars and the true molars are composed of two cres- 

 cents ; the anterior pillar is well developed, forming with the 

 metaconid the twin cusps (a a of Riitimeyer); the posterior 

 pillar which rises from the cingulum behind the entoconid is 

 present, but less advanced. The third molar has quite a large 

 fifth lobe or talon. 



In the recent Equidce, the most important change to be 

 noticed is that the teeth have become very long and prismatic, 

 and covered with cement. The incisors have deep pits of 

 enamel, and the pattern of the molars has become very complex, 

 through the sinuous folding of the enamel crests; nevertheless, 

 this complicated hypsodont molar is plainly derived from the 

 simple tooth of Mesohippus. " By tracing back the rise of the 

 eleven elements which compose the upper E quits molar, we find 

 that six belong to the primitive sextubercular bunodont crown. 

 Two elements of the ectoloph, the anterior pillar and median 

 pillar, rise from the simple primitive basal cingulum of the 



