37o UNGULATA 



Horse, where the teeth assume an extremely hypsodont form, the 

 original plan is so obscured by infoldings of the enamel that it can 

 only be traced with difficulty. 



At the present day the Perissodactyla are sharply differ- 

 entiated into Horses, Tapirs, and Rhinoceroses, but the knowledge 

 already gained of the extinct representatives of the suborder shows 

 such a close alliance between these groups that it is exceedingly 

 difficult to make any satisfactory classification of the whole. This 

 is of course exactly what might have been expected ; and the same 

 would doubtless be the case with all other groups if we knew as 

 much of their past history as we do of that of the Perissodactyles. 



The detailed account of the anatomy of the Horse given in the 

 sequel will afford much information as to the general structure of 

 the members of the suborder. 



Family Tapirid.e. 



Both upper and lower cheek-teeth brachydont and simply 

 bilophodont ; hinder premolars as complex as the molars ; last lower 

 molar without third lobe ; first upper cheek-tooth with a milk- 

 predecessor. 1 Outer columns of upper molars conical. Four digits 

 in the manus, and three in the pes. 



Tojnrus. 2 — Dentition i | c i p £, m § ; total 42. Of the 

 upper incisors, the first and second are nearly equal, with short, 

 broad crowns ; the third is large and conical, considerably larger 

 than the canine, which is separated from it by an interval. Lower 

 incisors diminishing in size from the first to the third ; the canine, 

 which is in contact with the third incisor, large and conical, working 

 against (and behind) the canine-like third upper incisor. In both 

 jaws there is a diastema between the canines and the commence- 

 ment of the teeth of the cheek -series, which are all in contact. 

 First upper premolar with a triangular crown, narrow in front 

 owing to the absence of the anterior inner cusp. The other upper 

 premolars and molars all formed on the same plan and of nearly 

 the same size, with four roots and quadrate crowns, rather wider 

 transversely than from before backwards, each having four cusps, 

 connected by a pair of transverse ridges, anterior and posterior. 

 The first loAver premolar compressed in front ; the others composed 

 of a simple pair of transverse crests, with a small anterior and 

 posterior cingular ridge. 



Skull elevated and compressed. Orbit and temporal fossa 

 widely continuous, there being no true postorbital process from 

 the frontal bone. Anterior narial apertures very large, and extend- 

 ing high on the face between the orbits ; nasal bones short, elevated, 



1 See W. N. Parker, Proc. Zool. Soc. 1SS2, p. 775. 

 - Cuvier, Tableau fiUment. de VHist. Nat. p. 152 (1798) ; ex Brisson. 



