PAL^OMASTODON. 



149 



number of smaller ones, wliicli 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. wintoni. The tibial surface (tib.) 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 (Jib.), and on its inner 

 side it is cut into by a deep rounded concavity (^.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 Palceomastodon (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. 



Eight astragalus of Palceomastodon : A, from above ; B, from below. 



ect., ectal facet ; fib., fibular facet ; Lm., 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 Tetrabelodon 

 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 {sus.) 

 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, which 

 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. 



