ORIGIN AND DEVELOPMENT OF THE EXISTING HORSES. 



723 



of the simple four-iobed pattern, resembling in this respect the suilline Artiodaciyla. 



The Diplarthra on the other hand are more numerous, and embrace a much 

 larger number of forms. They are divisible into ten families, including forty- 

 eight genera, variously distributed throughout geologic time. As but four of 

 these families concern us for the present, I will spare the memory of the reader 

 by not discussing the classification of the others. 



The first of all to which attention may be directed is the Lophiodoniidce, em- 

 bracing eight well defined genera which are not positively known to have existed 

 later than the upper Eocene epoch. They may be recognized, first, by the posses- 

 sion of four toes upon the anterior and three toes upon the posterior limbs; sec- 

 cond, by the molar and premolar teeth being different (Fig. 7), and third, by the 

 non-separation of the anterior and posterior external cusps of the superior molars 

 by an external rib-like pillar. The next family is the ChalicotheriidcB, to which 

 ten genera are referred. The digital formula is the same as in the Lophiodoniidce, 

 as is also the relation of the molar and premolar teeth. The only distinction is 

 found in the separation of the anterior and posterior external lobes by a vertical 

 ridge. The remains of this family range from the lower Eocene to the middle 

 Miocene, inclusive. The third family is the Falceotheriidce, having three toes upon 

 each foot. The molars and premolars are alike and the inferior molars possess 

 perfect double crescents. 



The fourth and last family is the existing Equidce, in which the digital for- 

 mula is reduced to one toe upon each foot. The molars and premolars are alike 

 and highly complex in structure. It is to this family all the existing horses be- 

 long, and it has been traced as far back as the upper Miocene strata. The rela- 

 tions in time of these families may be exhibited as follows : 



Having determined then the position that the horses hold in relation to'other 

 allied forms, as well as their geologic station, we are now prepared to trace the 

 history of their past succession, and point out the successive steps in modifica- 

 tion of structure by which one of the most specialized conditions known among 

 the ungulates has been reached. But before proceeding it may be well to under- 

 stand just here what is implied by this term specialized and what constitutes spe- 

 cialization in these forms. For the purpose of explanation I introduce a short 

 digression. 



