314 ON THE EXTINCT MAMMALIA OF 



Paeahippus cognatus. 



The Niobrara collection of fossils contains four specimens of molar teeth, apparently 

 of a solipedal animal of the family of Anchitheridce, differing from any of the pre- 

 ceding. The above name has been proposed for it, and, from the construction of the 

 teeth, it appears to have been more closely related to the Equicloi than any other of 

 its family. 



The teeth all belonged to the same individual, and appear to pertain to the tempo- 

 rary series. They are but slightly worn, are devoid of cementum, and have rugose 

 enamel, — much more so, indeed, than in any of the previously described fossils 

 belonging to the same family. In form, mode of insertion, and general constitution, 

 they bear a near resemblance to those of Anchitherium aurelianense, with which they 

 also agree in size. 



The specimens consist of the upper molar teeth of the left side and the first lower 

 molar of the right side. 



The superior molar teeth, represented in figure 7, plate XXI, have external demi- 



conoidal lobes as in Anchitlierium, but their outer face swells more gradually into a 



thicker and more prominent median ridge. The median lobes are proportionately 



more robust, and approach more equally in size the internal conical lobes. The 



median lobes are also thicker and more prominent at their inner extremity, and 



therefore less ridge-like and more conoidal than in Ancliitlierium. The outer extremity 



of the postero-median lobe divides into a pair of widely divergent processes, each of 



which sub-divides into a pair of smaller ones. These processes appear to correspond 



with the folds springing from the anterior horn of the postero-median column of the 



upper molars of the Horse, or those which complicate the contiguous borders of the 



central lakes of the worn triturating surface. The antero-internal lobe of the crown 



is more perfectly conical than in Anchitherium, and is proportionately somewhat 



smaller. The postero-internal lobe, relatively to that in advance, is rather larger 



than in the genus just mentioned. 



The first upper molar has its antero-posterior diameter proportionately greater than 

 the transverse diameter in comparison with the corresponding temporary tooth of 

 Anchitlierium, arising principally from the greater degree of development of the 

 antero-median lobe and the anterior accessory demi-conoidal lobe. The internal lobes 

 are more nearly equal in size than in the succeeding teeth. 



The first inferior molar, represented in figures 9, 10, plate XXI, resembles that of 

 Aiichitherium aurelianense in form and size, except that the internal median conical 

 lobe is larger, deeply notched, and cleft nearly to its base on the inner side. The 

 posterior tubercle springing from the basal ridge is also better developed. 



