190 The Palaeontological Record. I. Animals 



From their gregarious habits and individual abundance, the 

 history of many hoofed animals is preserved with especial clearness. 

 So well known as to have become a commonplace, is the phylogeny 

 of the horses, which, contrary to all that would have been expected, 

 ran the greater part of its course in North America. So far as it has 

 yet been traced, the line begins in the lower Eocene with the genus 

 Eohippus, a little creature not much larger than a cat, which has 

 a short neck, relatively short limbs, and, in particular, short feet, 

 with four functional digits and a splint-like rudiment in the fore-foot, 

 three functional digits and a rudiment in the hind-foot. The fore- 

 arm bones (ulna and radius) are complete and separate, as are also 

 the bones of the lower leg (fibula and tibia). The skull has a short 

 face, with the orbit, or eye-socket, incompletely enclosed with bone, 

 and the brain-case is slender and of small capacity. The teeth are 

 short-crowned, the incisors without "mark," or enamel pit, on the 

 cutting edge ; the premolars are all smaller and simpler than the 

 molars. The pattern of the upper molars is so entirely different 

 from that seen in the modern horses that, without the intermediate 

 connecting steps, no one would have ventured to derive the later 

 from the earlier plan. This pattern is quadritubercular, with four 

 principal, conical cusps arranged in two transverse pairs, forming 

 a square, and two minute cuspules between each transverse pair, 

 a tooth which is much more pig-like than horse-like. In the lower 

 molars the cusps have already united to form two crescents, one 

 behind the other, forming a pattern which is extremely common 

 in the early representatives of many different families, both of the 

 Perissodactyla and the Artiodactyla. In spite of the manifold 

 differences in all parts of the skeleton between Eohippus and the 

 recent horses, the former has stamped upon it an equine character 

 which is unmistakable, though it can hardly be expressed in words. 



Each one of the different Eocene and Oligocene horizons has its 

 characteristic genus of horses, showing a slow, steady progress in 

 a definite direction, all parts of the structure participating in the 

 advance. It is not necessary to follow each of these successive steps 

 of change, but it should be emphasised that the changes are gradual 

 and uninterrupted. The genus Mesohippus, of the middle Oligocene, 

 may be selected as a kind of half-way stage in the long progression. 

 Comparing Mesohippus with Eohippus, we observe that the former 

 is much larger, some species attaining the size of a sheep, and has 

 a relatively longer neck, longer limbs and much more elongate feet, 

 which are tridactyl, and the middle toe is so enlarged that it bears 

 most of the weight, while the lateral digits are very much more 

 slender. The fore-arm bones have begun to co-ossify and the ulna 

 is greatly reduced, while the fibula, though still complete, is hardly 



