340 



PRINCIPLES OF ANIMAL BIOLOGY 



of still another. Many living vertebrates have five digits on each hand 

 or foot, and there is anatomical and embryological evidence that primitive 

 vertebrates in general had five digits. These are numbered from the 

 inside outward, the thumb or great toe being first, the little finger or 

 little toe last. In the hind foot of Eohippus the functional toes are the 

 second, third, and fourth, while the fifth is reduced to a splint bone 



ah c 



Fig. 288. — Fossil teeth of ancient horselike animals, a, tooth of Eohippus with the 

 root broken; b, tooth of Mesohippus; c, tooth of Merychippus. {Photographed from 

 specimens in the Zoological Laboratory of the University of Michigan.) 



and the first is rudimentary. The forefoot had four functional digits, 

 the first being wholly wanting, though some old figures erroneously 

 include one. In the ancestors of the horse the first digits seem to have 

 disappeared first, followed by the fifth. The teeth of Eohippus were 

 short of crown and relatively long of root. The upper surface bore 



several conical cusps which, howevei-, 

 showed some sign of fusing to form 

 transverse crests (Fig. 288). The skull 

 (Fig. 289) was small, the lower jaw com- 

 paratively short, and the orbit Avas placed 

 well over the teeth, making the face 

 relatively short. Orohippus, which lived 

 in middle Eocene time, resembled Eohip- 

 pus closely but lacked the splint bone of 

 the forefoot (Fig. 290, left). 

 Mesohippus, an Oligocene form, was about 18 inches high. It had 

 only three digits on each foot (Fig. 290), but on the outer side of the 

 forefoot was a splint bone representing an extra toe (the fifth). Of the 

 three well-developed toes, the middle one (third) was in each foot dis- 

 tinctly larger than the others. The skull (Fig. 291), except for its 

 increase in size, had not changed materially. The cro\\Tis of the molar 

 teeth were still low (Fig. 288) and were tuberculate, that is, provided 

 with cusps on the upper surface, but the cusps were more distinctly 

 united into ridges or crests. Miohippus, a little later in Oligocene, was 

 somewhat larger, but otherwise much like Mesohippus (Fig. 290). 



Fig. 289.— Skull of Eohippus, 

 about ^io natural size. (From 

 model prepared by Ward's Natural 

 Science Establishment.) 



