HORSES. 487 



crowned molar of the extinct anchithere, while that of the latter does not differ 

 very widely from the molars of the rhinoceros represented on p. 481. Remembering 

 that the figured molar of the anchithere belongs to the opposite side of the jaws to 

 those of the horses, it will be apparent that it would only require a heightening of 

 its columns and ridges, accompanied by the formation of a series of foldings in 

 their investing enamel, and the filling up of the deepened intervening valleys 

 with cement, to produce a very similar type of tooth. It is almost superfluous 

 to add that the tall-crowned molars of the horse, with their completely filled 

 valleys, and their alternating ridges of harder and softer constituents, are far 

 more efficient instruments of mastication than the low-crowned teeth of the 

 anchithere, with their perfectly open valleys. Indeed, while the horse's are 

 adapted for a grinding action, and have nearly flat surfaces, the anchithere's 

 molars are suited to a champing motion, and have ridged surfaces. 



Another peculiarity in the dentition of the horses is that the 



incisor or front teeth in both jaws have an infolding of the enamel 



at the summit of their crowns, as shown in the figures A, B, C, on p. 490. 



pp 

 p -^KJSOSSti 



-.hy 



Tist^z^ r x^ 



Jiy p pi 



P 

 UPPER MOLAR TEETH OF THE ANCHITHERE (A), THE HORSE (B), AND THE HIPPABION (C). 



A is from the left, and B and C from the right side of the jaw. 



This peculiar structure, of which we shall have more to say in the sequel, may 

 be imitated by taking the finger of a glove and pushing in the top, and afterwards 

 filling the whole of the inside with wax. 



The skull of the horses differs from that of all other living Odd- 

 other characters. _ __ , , 1,1 1 ! 



toed Ungulates in having the socket of the eye completely surrounded 

 by bone. In all existing horses the number of toes on each foot is reduced to 

 one, which is enclosed in a large solid hoof. This toe, which corresponds in the 

 fore-limb to the human middle finger, is supported by a single long cannon-bone. 

 On the sides of this cannon-bone there are, however, small splints representing 

 the remnants of the second and fourth toes ; and in certain extinct forms (as 

 shown in the figures on p. 153) these lateral toes were complete and furnished 

 with hoofs, although they were much inferior in size to the middle toe, and 

 could have been of little, if any, functional importance. In defining the horse 

 family, it must accordingly be stated that although the toes may vary from 

 one to three in number, it is only the middle one that is functionally important. 

 Another distinctive feature of the family is that in the fore-limb the ulna is 

 represented only by its upper extremity, which becomes united with the radius ; 



