No. 2.] 
THE HARD PARTS OF THE MAMMALIA. 
261 
others, or are wanting. The development of V’s from these 
alternating cusps has then proceeded as in other types, with 
the exception peculiar to the Peris- 
sodactyla already referred to, that 
the external cusps only in both jaws 
are thus developed (Fig. 83). 
II. ORIGIN OF THE PROAL DENTI- 
TIONS. 
A. Rodentia. The phylogeny of 
the Rodentia as an order is now 
tolerably clear. I at first suggested,} 
and later asserted? that this order 
was derived by descent from the 
Tillodont suborder of the Bunothe- 
ria. The Tillodont suborder had a 
common origin with the Tzenio- 
donta, from some type of Bunothe- 
ria with unspecialized molars and 
premolars, in which some of the 
incisor teeth had begun to display 
enlarged size. A form allied to 
this ancestor is the genus Estho- 
nyx, which differs from it in but 
few respects. Professor Ryder, in 
discussing the origin of the Roden- 
tia,?> writes as follows: “The sig- 
nificance of accessory rudimentary 
incisors present in some forms of 
true rodents, as pointing to the man- 
ner in which the evolution of the 
rodent type of dentition took place, 
may be overrated; yet when it is 
borne in mind that in other groups 
the appearance of diastemata be- 
Figure 83.— Anchitherium equi- 
ceps Cope; superior and inferior 
molars in apposition, showing rela- 
tions of crests at different stages 
of mastication; one-half larger 
than nat. size; original. A, The 
last inferior molar beginning its 
transit across the superior molar, at 
the inner margin. 4, The inter- 
nal cusps of the former between 
those of the latter. C, The inter- 
nal of the inferior between the ex- 
ternal of the superior. 
tween the different kinds of teeth took place gradually, and in 
1 American Naturalist, April, 1883; report U.S. Geol. Surv. Tertiary Vertebrata, 
1885, p. 814. 
2 Loc. cit., April, 1884, 
3 Proceedings Academy Philadelphia, 1877, p. 317. 
