DAKOTA AND NEBRASKA. 297 



In the first molar, the anterior central lake of the triturating surface opens into the 

 oblique valley between the median and internal columns, as was described to be the 

 case in the anterior two temporary molars of both species of Merycldjrpus. The lakes 

 of the anterior two molars are only partially filled with cementum. 



The length of the space occupied by the six molar teeth is four inches and one 

 line. The measurements of the individual teeth are as follow : 



Length externally. 

 Length internally, 

 Antero-posterior diameter. 

 Transverse diameter, . 



2. Two superior molar teeth, apparently a fourth and the last of the series, from 

 the same individual. They are comparatively but little worn, and are x^epresented 

 in figures 49, 50, plate XVIII. They are smaller than the corresponding teeth of 

 Protohi])pus perditus, more robust, shorter, and more curved than those referred to P. 

 placidiis, and have about the same proportions as in the full series of MerycMppus 

 insignis above described, from the same locality. The worn triturating surfaces pre- 

 sent about as much simplicity in the arrangement of the constituent dental elements 

 as in the Horse, but rather more complexity than in the specimens oi Protoldppus. 

 The antero-internal column appears isolated, but presents a conspicuous process, 

 which at a later stage of abrasion of the teeth would evidently have become an 

 isthmus of conjunction with the antero-median column. 



The measurements of the specimens are as follow : 



4tli superior molar. 6tli. 

 Lines. Lines. 



Length externally, ...... 



Length internally, . 



Antero-posterior diameter, ..... 



Transverse diameter, ...... 



Antero-posterior diameter on same level as the more worn series 

 above described, ...... 



Transverse do., ....... 



While the transverse diameter of the two teeth is the same as that of the corres- 

 ponding teeth of the full series at the same level of the latter, the antero-postei'ior 

 diameter is greater. In the progress of protrusion of the crowns of the teeth in 

 equine animals, the antero-posterior diameter of the crown decreases, from disappear- 

 ance of the contiguous enamel due to pressure and friction of the teeth against one 

 another, while the transverse diameter remains unchanged. Hence, at a later age, 



38 



