DAKOTA AND NEBRASKA. 257 



SOLIDUNGULA. 



The Solidungula or Solipedia, an order whose sole existing representative is the 

 genus Equus, during the medial and later tertiary period down to the present epoch 

 appears to have been much richer in genera. According to the researches of the 

 author it was composed of two families, the Equidce or true Horses, and the Anchiihe- 

 ridce, and was represented by no less than eight distinct genera, inhabiting the 

 country of the United States. Of the eight genera, three only have heretofore been 

 observed in Europe and Asia, including the existing one, — namely, Equus, Hipparkm, 

 and Anchitherium. The additional genera have been named ProtoJiippus, Merychip- 

 pus, Hypohippus, Ancliipp>us, and Parahippvs. 



The Niobrara collection of fossils contains a multitude of well-preserved remains of 

 solipeds, consisting of teeth and bones and fragments of others, clear from matrix, 

 but very generally isolated and mingled pell mell in the utmost confusion, so that in 

 many cases I have found it impossible to refer them to any distinct species or genus. 

 Even some of those which have been referred to species, after much labor and com- 

 parison, may prove to have been incorrectly done. The separation of solipedal 

 remains under such circumstances is peculiarly difficult, on account of the close 

 resemblance between the bones and teeth of different species, as is exemplified in the 

 comparison of the bones and teeth of the different existing species oi Equus. 



EQUIDM. 



The Equidce are represented in the present condition of the earth by a single genus, 

 Equus, — at least so far as equine genera are distinguishable through characters de- 

 rived from the teeth. 



The superior molars of the Equidce have long square columnar ci'owns, which are 

 gradually abraded in the course of protrusion, and are inserted into the jaws by fangs 

 only after they are nearly worn out. The crowns are composed of six pairs of col- 

 umns with the intervals occupied with cementum, so that when the summits of the 

 columns are worn away the crown presents a broad masticating surface traversed by 

 lines of enamel, with alternating tracts of dentine and cementum. 



The external and median columns of the upper molars are the principal or larger 



ones, are of uniform size, and crescentoid in transverse section, with the contiguous 



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