990 : The Perissodactyla. 
cation of the feet ‘is of a different type, as already stated in the 
definitions of the two orders. oe 
e first step in dental modification in the superior series is 
the flattening of the external tubercles, and their connection 
with each other at the base. The 
beginning of this process is seen it 
Heptodon singularis (Fig. 4) and its 
allies of the Hyracotheriine.* A 
concomitant change is the confluence 
of the internal cusps with the inter — 
mediate ones into crests or ridges, 
which may be transverse or oblique 
(Figs. 4, 12, and 15), or may be other- 
wise varied, as in the equine line. 2 
the lower jaw two limes of changè 
have developed. In the one the tt- 
bone of Heptodon singularis percles of the crown have been of- 
Wasatch be 
of New Mexico, From Captain : : a 
Wheeler’sreport,iv.,ii.PLLXVI, Pairs by transverse ridges © he 
(Fig. 12); while in the other = 
cusps have become alternate, so that the ridges which joined 
them have been oblique, each tubercle giving origin to T 
crests extending in divergent directions,—that is, to the o 
with which it alternates. The result has been a W-shaped z 
or line of crests (Figs. 26 4, 28). 
Ryder haš pointed out that the rhinoceros 
(E. montanus, E. asinus, E. caballus), The same author 
out that the masticatory movement in the selenodont 
tyla is in the opposite direction, from within outware 
appears to me probable that many of the primitive eee od 
tyla had the same movement as the latter. They, wie 
sent a remarkable difference from the selenodont Artiot i 
in another respect. The mandibular condyle in Hy a 
* These are frequently accompanied by an additional external cusp, WHC 
from the anterior external angle of the crown. (See Figs. 4 4 
* Proceedings Academy Philada., 1878, p. 45. Although 
wards gave up this view, and concluded that the Artiodactyl 
‘ment as in the Rhinoceros (I. c., 1879, 47), his first opinion W 
correct one. 
as angot 
