IS 



KNOWLEDGE 



[Jakcaby, 1904. 



the tooth ; this pattern being a forerunuer of that obtain- 

 ing in the L-heek-t«eth of the horse. The long tail of 

 Hyracotheriiim Ts-as probably whip-like. Eemains of the 

 genus in question occur in the Lower Tertiary of North 

 America as well as in that of Europe. In somewhat later 

 deposits in both continents occur remains of more or less 

 closely allied mammals known as Packynohphus, which 

 mav or may not be in the direct horse ancestry. Later 

 still, the weU-known Palaeotheria of the Oligocene strata 

 of France and England, some of the species of which 



Fig. 3. — Crown Surfaces of a Bight Upper Cheektooth of Squiis 

 and of Two Eight T. pper Cheek-teeth of Anchitherium. 



were considerably larger than a tapir, were certainly off 

 the main line of descent, their structure approximating 

 more to that of the tapir type. When, however, we reach 

 the Miocene Tertiary of both hemispheres we come upon 

 remains of mammals which, although closely resembling 

 the palseotheria in dental structure, yet exhibit unmistakable 

 signs of nearer affinity with the horse. In Eumpe these 

 creatures are known as Anchitherium, but some of the 

 American forms are separated genericaUy as Miohippus, 

 one of the points of distinction being that whereas the 

 front, or incisor, teeth of the latter are of a perfectlv 

 simple structure, those of the former begin to exhibit a 

 slight infolding of the summit of the crown, thus fore- 

 shadowing the deep pit, or " mark," chai-acterisiag those of 

 the horse. The cheek-teeth oi Anchitherium (¥\g.'i),th<m^\i 

 still low-crowned, have acquired fully- developed transverse 

 crests, and a continuous outer waU. Numerically the teeth 

 agree with those of Syracotherium and Phenacodus, but a 

 difference is to be found in the relatively small size of the 

 first pair of cheek-t€eth iu each jaw. A marked advance 

 on the former is displayed in the fore-foot, which bv the 

 loss of the outer digit has once more become symmetrical, 

 with only three toes. In both limbs the cannon-bone 

 and toe- bones of the central digit have become greatly 

 enlarged at the expense of the lateral digits, which are 

 proportionately diminished, and there is a marked increase 

 in the relative lengths of all the bones of the lower portion 

 of the limbs. Moreover, it is noticeable that although the 

 radius and ulna in the second segment of the fore-limb, 

 and the tibia and fibula in that of the hind one remain 

 distinct from one another, yet the ulna and fibula have 

 become relatively more slender than in the earlier forms, 

 and are in places more or less welded respectively to the 

 radius and the tibia. In the matter of bodily size an 

 important advance has also been established, one of the 

 European species of Anchithe/nnm being approximately of 

 the dimensions of a tapir. In one of the American species 

 of the closely allied genus Mesohippug a remnant of the 

 upper end of the metacarpal, or uppermost bone, of the 

 outermost, or fifth, toe still persists. 



The next advance in this wonderful evolutionary chain 

 is presented by the members of the genus Protohippus, of the 

 Upper Miocene formation. These animals were essentiallv 



horses, although retaining the three toes of the ancestral 

 Anchitherium. The skull, for instance, had become 

 relatlvelv large and elongated, with the socket of the eye 

 separated from the temporal pit behind by a bony bar, and 

 thus enclosed by a complete ring of bone. The front, or 

 incisor teeth, were separated by an interval from the tusks, 

 or canines, wliich were relatively short, and divided by 

 another gap from the teeth of the cheek-series. Moreover, 

 the summits of the incisors were pushed in, like the ia- 

 turned fingers of a glove, thus giving rise to a distinct 

 "mark'' when half-worn. As regards the cheek-teeth, 

 those of the first, or " nulk " series, were curiously like the 

 permanent set in the Anchitherium. The second, or 

 persistent series, on the other hand, had acquired tall and,, 

 squared c'rowns, which only developed roots when the 

 animal was fully adult. In the pattern on the crown 

 these teeth closely resembled those of the modern horse, 

 with the exception of certain details which need not be 

 noticed here ; such pattern being the result of an excessive 

 elevation of the simple crests of the Anchitherivm molar, 

 coupled with the pushing-in of certain portions, and the 

 fiUing-up of the resulting hollows by the substance known 

 as " cement," which is altogether lacking in the former. 

 Then again, the first cheek-tooth in each jaw had become 

 small and rudimentary. In the feet the lateral toes, 

 although complete, had become relatively small, and 

 scarcely, if at all, reached the ground, being in fact 

 analogous to the rudimentary lateral toes of the ruminants. 

 On the other hand, the central toe in each foot, with its 

 supporting cannon-bone, was proportionately enlarged, 

 and had become the real support of the body ; the animal, 

 like the modern horse, apparently standing solely on the 



Fig. 4. — Skeleton of Left Hind and Fore Feet of Protohippus. 



tenninal joint of its middle toes. Higher up, the ulna in 

 the fore-liml), and the fibula in the hind one, had become 

 imperfect. 



In the ordinary Pliocene three-toed horse, or Hipparion, 

 of Europe and Asia, together with its North American 

 representative, separated by some naturalists under the 

 name of Neohipparion, the lateral toes were quite function- 

 less, the ulna in the fore-limb had become fused with the 

 radius, and the fibula in the hind-limb with the tibia, 

 while the cheek-teeth had acquired somewhat taller and 



