SUBORDER c PERISSODACTYLA 135 



the molars ; in all later rhinoceroses, the premolars and molars, with the 

 exception of the most anterior premolar, exhibit essentially the same com- 

 position. Thus, from the transverse ridges of the superior molars (Fig. 172) 

 several folds jDroject into the median valley ; according to Osborn's termino- 

 logy, one from the ectoloph (crista), one from the protoloph (antecrochet), 

 and one from the metaloph (crochet). These folds sometimes unite, enclosing 

 island-like spaces or fossettes. The last inferior molar never has a third lobe. 



The skull is low, elongated, occiput surmounted by a sharp occipital crest. 

 The orbits are open posteriorly, and the temporal fossae are unusually large. 

 The nasal bones stand out freely, have very diverse 



length and stoutness, according as they bear horns E ^^^. , _^ 



or are hornless, and are sometimes supported by a _ ^-^ \ ~JJr~^:^l^^ 

 co-ossified mesethmoid. The unusually large narial \\.^-^c-^ ^ i 

 apertures often extend back as far as the first molar. // /«^P) (Wx®/'Xa 

 The well-developed postglenoid process is either iL^/ '' />^¥ '^-7/-' ^ 

 separated from the mastoid process (post-tympanic) M^M~^:^!!^mi^i^yJr p 

 by a groove or is united with it. ^^^m^^I 



The extremities are usually short and stout. | !^|f^^W 



The radius and ulna are well developed and entirely /iiw ^Mn 



distinct, as in the tapir. In the older forms, the L^ ^ 



carpus exhibits nearly the same construction as in fig. ii2. 



the tapir. Of the four metacarpals, the third is Antecrochet T t'. crochetT Cr, 

 stouter than the two adiacent ones, while the fifth *:nstai^''5'r*'°V'' = ^^'J"l^^^^ 



J ' P, protoloph ; Fs, parastyle. Vs- 



IS short. In the later forms, the manus has but 



three toes ; the carpals and metacarpals become shorter and broader, and the 

 third metacarpal is considerably stouter than the two lateral metacarpals. 

 The femur is always characterised by a strongly marked third trochanter, 

 situated rather low down on the shaft. In more primitive forms, the tarsus 

 and metatarsus are somewhat elongated and narrow ; in more modern types, 

 they are short and broad. 



Subfamily 1. Htracodontinae Cope. 

 Skull short, with a sagittal crest and with periotic conspicuous laterally. Nasal 



q -| j^ o 



hones projecting freely, hornless. Dentition complete : ' ' ' ' • Canines weak 



O. i . 4:. O. 



and immediately following the chisel-shaped incisors, hut separated from the cheek 

 teeth hy a short diastema. Premolars and molars either heterodont or homoeodont, the 

 superior molars heing composed of an ectoloph and two ohlique transverse crests, the 

 inferior consisting of two angular crescents, the posterior horn of which forms a trans- 

 verse crest. Neck long. Extremities long and slender. Manus with three or four 

 digits ; pes tridactyl. 



In their general proportions, these slightly built animals, with their long 

 limbs and long and slender neck, bear a far closer resemblance to the horse 

 or Anchitherium than to Rhinoceros, although the skull and cheek teeth agree 

 more closely in structure with those of the latter genus. They approach the 

 tapirs in the skeleton structure, and form an independent extinct lateral 

 branch of the rhinoceros stem. They are at present known only from the 

 Eocene and Oligocene of North America. 



