No. 2.] THE HARD PARTS OF THE MAMMALY/A. 177 
dently is the inward flexion of the manus, and it is especially 
characteristic of forms which suspend themselves by the fore 
limbs, as above pointed out. It is not difficult to perceive in 
this instance how the function has produced the structure by 
use. Continued flexion and extension has rounded the angles, 
especially on the dorsal side of the carpus, and the stimulation 
has elongated the elements in the direction of greatest pulling 
strain; z.e. longitudinally (Fig. 20). This is in great contrast 
to the flatness, both transversely and anteroposteriorly, seen in 
Artiodactyla and horses, where the parts have been subjected to 
impact from hard earth for so long a period. 
Some apparent exceptions to the above general principles 
must be now referred to. The genera Hyrax and Dendrohyrax 
are closely allied, yet the one displays a taxeopod, and the latter 
a diplarthrous tarsal structure (Osborn). A preparation for the 
latter is seen in some Periptychidz, and in some Carnivora, as 
Ursus and Mustelidz, in the extent of the contact of the condyle 
of the head of the astragalus with the cuboid, without the produc- 
tion of a facet on either. This facet could be, however, easily 
produced by the simple flattening of the appressed surface by 
impacts, and this is, it may be supposed, what has taken place 
in Dendrohyrax. Observations on the movements of the spe- 
cies of this genus in life are necessary to solve this question. 
The Amblypoda of the Wasatch and Bridger epochs are dis- 
tinguished by the great diplarthrism of the tarsus, although 
they are plantigrade and pentadactyle (Figs. 8-10). In this order, 
as already indicated, the feet have shortened with the advance 
of time; and though it is hardly probable that they have 
descended from a digitigrade type, their ancestor, Pantolambda, 
was much more capable of varied movements than they. The 
history of this line must be better known before any reasonable 
explanation of the character of the hind foot can be reached. 
4. THE GINGLYMOID ARTICULATIONS. 
The mobility of the ginglymoid articulation depends on its 
combination of convex with concave surfaces, which permit 
flexion and extension of one bone on another; in other words, 
they consist of a condyle and an adapted cotylus. The fixed 
articulations consist, on the other hand, of adapted faces, which 
