PAL^OMASTODON. 
149 
number of smaller ones, which appear to have belonged to Proboscideans, occur in the 
collections. The largest of these is shown in text-figure 52, and may be referred pro- 
visionally to P. imntoni. The tibial surface {tih.) is comparatively narrow from before 
backwards and is strongly and evenly convex in the same direction ; at its outer edge 
it is continuous with the nearly vertical surface for the fibula {,fih.), and on its inner 
side it is cut into by a deep rounded concavity [i.m.) looking upwards and inwards, 
evidently for the reception of a large internal malleolus such as has been described 
as occurring in the tibia of Palwomastodon (see above, p. 147). Behind this the 
postero-internal angle of the bone is produced into a strong tuberosity which in some 
of the smaller specimens forms a pedunculate knob. In front of the tibial surface the 
anterior face of the bone is concave and forms a short neck for the prominent rounded 
Text-fig. 52. 
2 . 
5 . 
Eight astragalus of Palaomastodon : A, from above ; B, from below. 
eet., ectal facet ; Jib., flbular facet ; i.m., surface for internal malleolus of tibia ; nav., navicular facet ; 
sus., sustentacular facet ; tib., tibial facet. | nat. size. 
navicular facet {nav.), which is almost precisely as in the astragalus of Tetrahelodon 
angustidens. There are two calcaneal surfaces : of these the ectal {ect.) is flat and 
nearly triangular in outline, the apex being anterior, while the sustentacular (sms.) 
is an elongated oval facet with the long axis antero-posterior, in which direction it is 
deeply concave. The anterior edge of the sustentacular joins the lower edge of the 
navicular facet. The two surfaces are separated posteriorly by a deep groove, wdiich 
widens out in front into a broad fossa, the anterior border of which is formed by the 
lower edge of the navicular surface. The chief differences between this astragalus 
and those of the later Proboscidea are: (1) the tibial surface is narrower and at the 
same time more strongly convex ; (2) there is a large internal malleolus in the tibia 
involving the presence of a deep concavity in the astragalus for its reception; (3) the 
neck is somewhat better developed, at least in the smaller species ; (4) the postero- 
internal angle of the bone is very prominent. 
