No. 2.] THE HARD PARTS OF THE MAMMALIA. 189 
IJ. Looking upwards. 
6. The lateral proximal astragalar. 
7. The proximal phalangeal. 
I premise that the elbow joint comes under this head in the 
higher Diplarthra, but not being generally of this character, it 
has been already considered elsewhere (p. 179). 
The trochlear keels which look downwards are much the 
most prominent and important. They are all projections from 
the ends of the respective elements. The up-looking are gener- 
ally projections of the edges of bones. Such are the lateral 
crests of the astragalus, and of the phalanges of the Edentata. 
The tongues of the tongue-and-groove articulations exhibit vari- 
ous degrees of development in the different Mammalia. Those 
of different parts of the skeleton coincide in their condition in 
any one type of ambulatory Mammalia, and so may be all con- 
sidered together. This fact suggests strongly that they are all 
due toacommon cause. The 
proximal ridges of the pha- 
langes are very weak except 
Fic. 27. Fic. 28. 
Figure 27.— Right posterior foot of a species of Coryphodon from New Mexico, 
one-half nat. size; from Report Expl. W. of tooth Mer., G. M. Wheeler, IV., 
Pl exe 
Figure 28.— Right posterior foot of Afphelops megalodus Cope, from Colorado, 
one-half nat. size; from Report U. S. Geol. Surv. Terrs., F. V. Hayden, IV., 
Pl. CXXX., 
