HORSES. 
91 
mammals, there being only a single toe on eaeh 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 Artiodactyla, 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 mammaks 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 and lumbar vertebrse 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. [Qase 13 ] 
The former (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. §, C. Pm. M. § 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. 
