EXTINCT FORMS. 



5i5 



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

 the lateral toes of the anchithere, as shown in the figures on p. 153, 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 " character- 

 istic of the incisors of the horse was but faintly indicated in one species alone. 

 Passing downwards 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 comparison of the figure of the molar of the anoplothere on 

 p. 421, with that of the anchithere on p. 487. 



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

 each foot, as shown in the figure on p. 8 of 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 generalised type of structure, 

 appears then to be the undoubted ancestral stock from which the modern horse 

 has been slowly produced by some process of evolution, which was going on through- 

 out 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. 



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



LopModons. ancestral line of the modern horse, there were a number of other more 

 or less closely-allied types belonging to the Odd-toed group. Among these some of 

 the best and longest 

 known are the palaso- 

 theres, from the upper 

 Eocene strata of Europe, 

 of which, as far back as 

 the early portion of the 

 present century, nearly 

 complete skeletons dis- 

 covered in the gypsum 

 quarries, near Paris, 

 were described by Cuvier. These paiaeotheres were tapir-like animals, with three 

 toes to each foot, and molar teeth approximating to those of the anchithere in 

 structure, but having a somewhat elongated neck. While some of the species were 

 not taller than a sheep, others must have fully equalled the largest tapirs in size. 

 They probably resembled the tapirs in having a short proboscis to the snout, and 



THE LEFT UPPER CHEEK-TEETH OF THE GREAT PAL.EOTHERE 



(h nat. size). — After Gaudry. 



