192 The Palaeontological Record, I. Animals 



mals was then so small, that Cuvier's types were forced into various 

 incongruous positions, to serve as ancestors for unrelated series. 



Tlie American family of the Titanotheres may also be distantly 

 related to the horses, but passed through an entirely different course 

 of development. From the lower Eocene to the loAver sub-stage of 

 the middle Oligocene the series is complete, beginning with small and 

 rather lightly built animals. Gradually the stature and massiveness 

 increase, a transverse pair of nasal horns make their appearance and, 

 as these increase in size, the canine tusks and incisors diminish 

 correspondingly. Already in the oldest known genus the number 

 of digits had been reduced to four in the fore-foot and three in the 

 hind, but there the reduction stops, for the increasing body-weight 

 made necessary the development of broad and heavy feet. The final 

 members of the series comprise only large, almost elephantine animals, 

 with immensely developed and very various nasal horns, huge and 

 massive heads, and altogether a gi-otesque appearance. The growth 

 of the brain did not at all keep pace with the increase of the head 

 and body, and the ludicrously small brain may well have been one of 

 the factors which determined the startlingly sudden disappearance 

 and extinction of the group. 



Less completely known, but of unusual interest, is the genealogy 

 of the rhinoceros family, which probably, though not certainly, was 

 likewise of American origin. The group in North America at least, 

 comprised three divisions, or sub-families, of very different pro- 

 portions, appearance and habits, representing three divergent lines 

 from the same stem. Though the relationship between the three 

 lines seems hardly open to question, yet the form ancestral to all 

 of them has not yet been identified. This is because of our still very 

 incomplete knowledge of several perissodactyl genera of the Eocene, 

 any one of which may eventually prove to be the ancestor sought for. 



The first sub-family is the entirely extinct group of Hyracodonts, 

 which may be traced in successive modifications through the upper 

 Eocene, lower and middle Oligocene, then disappearing altogether. 

 As yet, the hyracodonts have been found only in North America, and 

 the last genus of the series, Hyracodon, was a cursorial animal. 

 Very briefly stated, the modifications consist in a gradual increase 

 in size, with greater slendcrness of proportions, accompanied by 

 elongation of the neck, limbs, and feet, which become tridactyl and 

 very narrow. The grinding teeth have assumed the rhinoceros-like 

 pattern and the premolars resemble the molars in form ; on the 

 other hand, the front teeth, incisors and canines, have become very 

 small and arc useless as weapons. As the animal had no horns, it 

 w.as quite defenceless and must have found its safety in its swift 

 nnming, for Hyracodon displays many superficial resemblances to 



