276 ON THE EXTINCT MAMMALIA OF 



The general construction of the teeth, — that is to say, the arrangement of the 

 enamel in its relation with the other elements, — as seen in the abraded surfaces, 

 represented in figure 2, plate XVII, is nearly the same as in the Horse. The median 

 lakes are more gaping or relatively more capacious than in the latter, even at the 

 same stage of attrition, and the enamel border is rather simpler in its course or less 

 folded. 



The most important differences observable in the teeth of the fossil from those of 

 the Horse and Ass, — those, indeed, upon which I have looked as of generic value, — 

 are found in the arrangement of the internal columns of the crown. In the genus 

 JEgims the antero-internal column expands both before and behind its connection with 

 the antero-median column, and in transverse section, as observed in the triturating 

 surface, presents an elongated antero-posterior ellipse. In Protohippus, the new genus 

 to which the fossil has been referred, the antero-internal column of the crown extends 

 only backward from its connection with the antero-median column, and in section 

 presents a short oblique ellipse, much as is the case in the upper temporary molars 

 of the Horse. 



In the latter, the postero-internal column of the upper molars preserves its dis- 

 tinctness from the one in advance and from the postero-median column, until the 

 crown is nearly worn away to the fangs. In Protohippits the fold or inflection sepa- 

 rating the postero-internal and postero-median columns extends but a comparatively 

 short distance in the length of the crown, so that it becomes early obliterated; just 

 previously leaving a small elliptical ring on the triturating surface. 



Dr. Lund's figures 1 and 3, plate xlix, of the Royal Danish Academy of Sciences, 

 representing upper molars of Equus principalis and E. neogceics, and M. Gervais' 

 figure 1, plate vii, of the Researches on the Fossil Mammals of South America, repre- 

 senting upper molars of E. neogceus or E, macrognathus, but to which I have pre- 

 viously referred as probably pertaining to E. p>rincipalis, exhibit the same peculiarity 

 of arrangement in the enamel on the triturating surface, and therefore likewise 

 pertain to the genus ProtoM'ppus. 



Measurements of the fossil in question, in comparison with those of the correspond- 

 ing part of the recent Horse and Ass, are as follow : 



Length of space occupied by back four molars, 

 Antero-posterior diameter of third molar. 

 Transverse do. (independent of cement), 

 Antero-posterior diameter of last molar, 

 Transverse " " 



Distance from alveolar edge to infra-orbital margin at 



back of last molar, .... 20 29 42 



perditus. 



E. asinus. 



E. 



caballus 



Lines. 



Lines. 





Lines. 



38 



43 





51 



10 



12i 





13 



10 



11 





13 



n 



lOi 





15 



8J 



8 





11 



