HORSES. 91 



mammals, there being only a single toe on each foot (fig. 49) ; but 

 in their ancestors, as shown in the Geological Guide, p. 23, the 

 toes are present in greater numbers as we go back in time. Their 

 molar teeth are large, quadrangular, and highly complex, the 

 enamel-foldings being extremely numerous when compared with 

 the simpler ones of the Rhinoceros and Tapir. In their skulls 

 and teeth all the recent Horses and Asses show scarcely any dif- 

 ference, the species being merely separated by size, form of tail, 

 colour, and other external characters. 



Suborder ARTIODACTYLA. 



The Artwdactyla, or Even-toed Ungulates, are so called because 

 their feet always possess an even number of toes, two or four, the 

 centre line of the foot passing down between the toes which 

 correspond to the third and fourth of the complete or typical five- 

 toed mammal's foot ; these two toes are always equal, and larger 

 than the second and fifth, if these are present ; the first, corre- 

 sponding to our thumb, is (as in existing Ungulates generally) 

 always absent. The metacarpal and metatarsal bones of the third 

 and fourth digits are generally united, and form what is known as 

 the " cannon bone." The premolars and molars are quite distinct 

 in shape, the former being single- and the latter two-lobed. The 

 dorsal arid lumbar vertebrae together invariably number nineteen. 



The Artiodactyles may be, as already noticed, p. 38, divided 

 into two groups, Non-ruminants and Ruminants groups charac- 

 terized also by their dentition, the teeth of the former being 

 covered with blunt, rounded cusps, and those of the second having 

 two pairs of crescentic ridges on their surfaces. 



The non-ruminating Artiodactyles are Hippopotami and Pigs. [-Q ase 13 1 

 The foruier (Case 13) possess broad skulls, huge lower jaws, and 

 great canine teeth, which, if the corresponding tooth in the opposite 

 jaw is lost, will continue to grow, forming great outwardly curved 

 tusks, such as the one placed in Div. B. The molars are large, 

 square, and complex. The dental formula of the common Hippo- 

 potamus, of which a skeleton stands in the centre of the Gallery, is 

 I. f , C. j, Pm. f , M. f x 2 = 40 ; the skull of a fine skeleton of the 

 diminutive Liberian Hippopotamus exhibited in this case shows 

 only one lower incisor on the left side, but two on the right. 



