j 1887] The Perissodactyla, . 989 
sion. Hence no alternation of these bones takes place in the 
hind foot of the Carnivora, where the eversion of the digits is 
moderate. In the case of the fore foot, the eversion and conse- 
quent torsion are so great that the alternation is produced (Plate 
_ XXVIII). Inthe manus of the plantigrade bear the alternation 
is almost #727. con 
It may be here objected that the camel walks upon elastic pads 
= asdo the Carnivora, and yet the alternation does really take place. 
lt is on this account (as I have maintained) that the distal meta- 
podial tongue-keels were never completed in these animals. But 
if the camel does not rest on the ungues sufficiently to resist 
torsion, as in the Carnivora, this was not the case in the ances- 
tors of the camels, the Poëbrotheriidæ, where the contact with 
the ground was much as in other Diplarthra. But there is little 
impact in the step such as occurs in other Ungulata, hence the 
failure to complete the keels. 
Our zoological gardens furnish excellent opportunities for the » 
i ‘erage of all these observations on the progression of Mam- 
ut: 
I. PERISSODACTYLA. 
The modifications of structure seen in the evolution of this 
sub-order are principally those of the feet, teeth, and vertebræ. 
already remarked, the changes in the feet consist in the 
sradual reduction of the number of the digits from four, or per- 
PS five, to one. In the teeth the changes consist in the con- 
Version of tubercular, or bunodont, into crested, or lophodont, 
3 molars, and in the increase in the complexity of the premolars, 
_ *° that, instead of possessing a distinct and simple character of 
T own, they come to resemble the true molars; also in the re- 
4 duction in the dimensions and numbers of the canine and incisor 
= in some lines, The modification of the vertebra consists 
3 the conversion of a flat zygapophysial articulation into a con- 
_ “vo-convex or embracing structure. This modification is sim- 
T what is seen in some of the Artiodactyla, but the change 
i eee further, and the structure becomes more complex in 
: io = Suborder.” In the teeth the change pursues two paths, 
"A different from that seen in the Artiodactyla. The modifi- 
z 
¢ Pag a ote Arrangement and Phylogeny of Artiodactyla,” in Proceeds. 
~ S. Society, September, 1887. 
