No. 2.] THE HARD PARTS OF THE MAMMALIA. 197 
of the distal groove so as to render the articulations, tongue and 
groove, like the tibio-astragalar articulation of Diplarthra. This 
structure gives great strength, and an increased protection 
against dislocation. It exists in both feet of the Manidz and 
Myrmecophagide, and in the anterior digits of the Dasypodidze 
which have especially developed ungues (Figs. 37-39). The fact 
that the ungues differ in this respect in the armadillos, e.g. 
Priodontes maximus, gives us an excellent opportunity for inves- 
tigating the origin of the structure. The first and second digits 
of the fore foot present perfectly simple phalangeal articulations. 
The ungues are slender, and from their position take no part in 
excavation of the earth. The ungues of the third and fourth 
digits are enormously developed, and are chiefly used in excava- 
tion. The phalanges display the tongue-and-groove joints. The 
ungues of the posterior foot resemble hoofs in form and func- 
tion, and the phalangeal articulations are flat and simple, greatly 
resembling those of primitive ungulate Mammalia, except that 
they have no trace of keel posteriorly. The close relation of 
function to the structure is here obvious. The deep tongue-and- 
groove articulation is a consequence of the excessive use of the 
digits which exhibit it, in the excavation of the earth. The 
mechanical cause of the structure must be in the main similar 
to that which gave origin to the tibio-astragalar articulation of 
the Diplarthra, but it has acted in the reversed direction ; that 
is, the median groove looks downwards and the keel looks up- 
wards. This difference is to be traced to the different condi- 
tions under which the respective processes commenced. In the 
case of the tibia, the extremity was primarily convex downwards, 
owing to the codperation of impact with gravity. The distal 
extremity of the phalange was originally concave, owing to the 
pressure of a median tendon and sesamoid bone. In each case 
the continued application of the strain would tend to exagger- 
ate the original condition. After a certain extent of arc of the 
phalangeal articulation has been reached, the sesamoid bone has 
little effect, and the continued growth of the keels must be due 
to continued impacts combined with torsions of the digits ; the 
impacts affecting most energetically the middle line. The tor- 
sion movement would have the effect to compress the keels ex- 
actly in the line or arc of flexion and extension, by alternate 
