PERISSODACTYLA. 



h'J'.^ 



considerably from the equine type of pattern and resemble tlios:; of 

 Palieotheriuni whicli is the most tapir-like. 



Fam. 4. Hyracotheriidae.* i | c i p \ m | ; grinders low and 

 tubercular, premolars (usually tritubercular) simpler than the molars ; 

 upper molars (Fig. 317, A) with six cusps, lower (Fig. 317, -B) with four ; 

 tubercles conical or V-shaped, ridges low ; orbit not closed, but post- 

 orbital process of frontal present ; odontoid conical ; radius and ulna 

 subequal, separate ; scapula with a well-marked coracoid process ; manus 

 with four (without trace of No. 1), pes with three digits. They are the 

 oldest Perissodactyls and are confined to the Eocene of Eur. and N. 

 Amer. Their grinders are practically bunodont, at any rate in the upper 

 jaw where there are hardly any outer wall or ridges. Hyracotherium Ow. , 

 a smallish animal, about 3 feet long. Lower Eocene of Eur., and N. Amer. ; 

 Eohippus Marsh, and Protorohippus Wort., Eocene, N. Amer. ; Orohippus 

 (Epihippus) Marsh, and Pachynolophus Pomel, Eocene of Eur. and Amer. 

 teeth more equine with ridges connecting the tubercles ; Propalaeo- 

 therium Gerv., M. Eocene, Eur., etc. 



Much has been written on the ancestry 

 of the horse. It has been maintained by 

 many authors that a continuous series of 

 forms connecting it with the four-toed, 

 brachyodont, bimodont Hyracotheridae 

 of the Eocene has been discovered and 

 that here if anywhere a demonstrative 

 historical proof has been obtained of the 

 truth of the doctrine of organic e^-olution. 

 Without desiring in the smallest degree to 

 impugn that doctrine, it may be permitted 

 us here to examine rather closely the 

 view that the series of forms which recent 

 palaeontological research has undoubtedly 

 brought to light constitute that historical 

 proof which has been claimed for them. 



The forms which are vitihsed for this series are : Pliohijypus, Protohippus, 

 Desmathippus, Miohippus, Mesohippus, Orohippus and Hyracotherium. 

 The characters which are chiefly pointed to as showing the gradation are 

 those of the limbs, and the teeth, and to a certain extent of the skull. 

 Beginning at the lower end of the series, we find in Hyracotheriidae, brachy- 

 odont molars which are practically bunodont, a complete dentition, pre- 

 molars simpler than the molars, a well marked coracoid process on the 

 scapula, a conical odontoid process, an orbit not closed in by bone 

 though the frontal has a postorbital process, four complete digits on the 

 manus and three on the pes, radius and ulna nearly equal in size and 

 separate. In the stage next succeeding in the ascending order — Palaeo- 

 theriidae—we find these characters modified as follows : the teeth are still 

 brachyodont wdth little cement, low ridges connect the tubercles, the 

 hinder premolars are usually like the molars, a complete dentition ; the 

 orbit is still open ; the radius and ulna are still separate and about equally 

 developed, but the manus is tridactyle, digit No. 5 having become reduced 



FiO. 317. — Hyracotherium lepor- 

 inum Ow. A second upper, B 

 first lower molar. The tubercles 

 are : a antero-external, a' acces- 

 sory, 6 postero-external, 6' an- 

 tero-intermediate, 6" postero-in- 

 termediate, c postero-external, 

 d postero-internal ; p antero- 

 external, ^1 antero-internal, y 

 postero-external, y^ postero-in- 

 ternal (from Zittel, after Owen). 



* Wortman, Species of Hyracotherium^ etc., Bull. Amer. Mus. Nat. 

 Hist., 8, 1896, p. 81. Earle, Comparison of the American and European 

 forms Hyracotherium, Amer. Nat. 1896, p. 131. 



