OTHER EXTINCT ODD-TOED UNGULATES 1103 



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OTHER EXTINCT ODD-TOED UNGULATES 



Ancestry of the ^e fore g in g observations indicate that there is a complete transi- 

 Horse ^ on f rom the modern single-toed horse to species with three distinct 

 toes to each foot, and with rather shorter-crowned and simpler molar 

 teeth. From these three-toed horses there is a further gradation to other extinct 

 Ungulates, which cannot be included in the Equine family, but some of which were 

 doubtless the direct progenitors thereof. One of these was the Miocene anchithere, 

 common to both Europe and the United States. From the figures given on p. 1075, 

 it will be seen that the upper molar teeth of these animals, although formed on the 

 general plan of those of the horse, have very low crowns, with a simpler arrang- 

 ment of the pillars and ridges, and the intervening valleys perfectly open, owing to 

 the absence of cement; and it may be added that other species show a complete 

 transition from the molars of the anchithere to those of the earlier horses. Farther, 

 the lateral toes of the anchithere, as shown in the figures on p. 743, were relatively 

 larger than in the three-toed horses. Moreover, in the anchithere, the radius and 

 ulna in the fore, and the tibia and fibula in the hind-limb, were perfectly distinct and 

 fully-developed bones. The largest anchithere approached an ordinary pony in size, 

 while the smallest was not larger than a sheep, and in all these animals there was 

 the full typical number of forty-four teeth, while the " mark" characteristic of the 

 incisors of the horse was but faintly indicated in one species alone. Passing down- 

 ward in the geological scale, by a complete transition from the anchithere, we arrive 

 in the lower Eocene London Clay at a small animal known as the hyracothere, 

 which was not larger than a fox, and had four toes to the front, and three to the 

 hind-feet; while the forty- four low-crowned teeth were of still simpler structure 

 than in the anchithere, although formed on the same general plan. The last lower 

 molar tooth of the hyracothere differs however from that of all existing Odd-Toed 

 Ungulates in having three complete lobes, and thus approximates to the corresponding 

 tooth of the Even-Toed group; and it may be added that the essential correspondence in 

 the structure of the upper molars of the two groups will be apparent by a compari- 

 son of the figure of the molar of the anoplothere on p. 1008, with that of the anchi- 

 there on p. 1075. 



A step from the hyracothere brings us to the still earlier phenacodus, in which 

 each foot as shown in the figure on p. 8 in the first volume, had five complete toes; 

 while the molar teeth had their crowns with small isolated tubercles instead of ridges. 

 This small primitive animal, with a most generalized type of structure, appears then to 

 be the undoubted ancestral stock from which the modern horse has been slowly pro- 

 duced by some process of evolution, which was going on throughout the long ages 

 of the whole Tertiary period, and it is at least noteworthy that the true horse only 

 made its appearance on the globe at or about the same time as his master, man. 



In addition to the animals referred to above, as forming the direct 



Palaeotheres ancestra i ij ne o f t h e modern horse, there were a number of other more 

 and Lophi- 

 odons or ^ ess closely-allied types belonging to the Odd-Toed group. Among 



these some of the best and longest known are the palaeotheres, from the 

 upper Eocene strata of Europe, of which, as far back as the early "portion of the 



