PERISSODACTYLA. 



599 



considerably from the equine type of pattern and resemble those of 

 Palaeotherium which is the most tapir-Uke. 



Fam. 4. Hyracotheriidae. 



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. Hyracotheriwn Ow, , 



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



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



(Epihippus) INIarsh, 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, 



brachj'odont, bunodont Hyracotheridae 



of the Eocene has been discovered and 



that here if anywhere a demonstrative 



liistorical proof has been obtained of the 



truth of the doctrine of organic evolution. 



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 wliich recent 



palaeontological research has undoubtedly 



brought to light constitute that historical 



proof wliich has been claimed for them. 



The forms wMch are utilised for this series are : PlioUppus, ProtoUppus, 



Desmathippus, Miohippus, Mesohippus, Orohippus and Hyracotheriwn. 



The characters which are chiefly pomted 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 bimodont, 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, fom- complete digits on the 

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

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

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

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

 liinder premolars are usually hke the molars, a complete dentition ; the 

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

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



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

 Hist., 8, 1S96, p. 81. Earle, Comparison of the American and European 

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



Fia. 317. — hyracotherium lepor- 

 ilium Ow. A second upper, B 

 first lower molar. The tubercles 

 are : a antero-external, o' acces- 

 sory, & postero-external. 6' an- 

 tero-intermediate, b" postero- in- 

 termediate, c postero-external, 

 d postero-internal ; p antero- 

 external, i3' antero-internal, y 

 postero-external, y' postero-in- 

 ternal (from Zittel, after Owen). 



