54 TEETIAET VEETEBEATA OE THE FATtTM. 



a shelf-like projection bearing on its upper surface a facet (t.) for articulation 

 with the fibular facet of the tibia above described. Beneath this facet the distal 

 portion of the fibula forming the external malleolus projects considerably, and its outer 

 face is swollen and roughened, while its inner, nearly triangular surface bears two 

 facets almost in the same plane. Of these facets the upper (ast.) is the smaller and 

 articulates with the astragalus, so that when the tibia and fibula are in their natural 

 position with regard to one another the astragalar facets of the two bones form a 

 continuous concave surface. The lower and larger facet (calc.) articulates with the 

 large fibular facet of the calcaneum, which lies immediately external to the ectal facet 

 for the astragalus. The astragalar and calcaneal facets of the fibula are only separated 

 by a slight ridge. 



The fibula, particularly its distal portion, is extremely like that of Elephas. In 

 both there is the same concavity of the inner face of the distal end, the prominent 

 facet for articulation with the tibia, and an almost identical arrangement of the 

 surfaces for the astragalus and calcaneum. The fibula of Uintatherium is different 

 in all these particulars, more especially in the very small size or even complete 

 absence of the calcaneal facet; it should, however, be remarked that in the earlier 

 Amblypoda, e. g. Coryphodon, the calcaneal articulation of the fibula is well 

 developed, and this is no doubt a primitive feature. 



Examples of the astragalus (text-fig. 32, 2 a, 2 b, 2 c) are among the most numerous 

 of the bones found in these deposits, and it is remarkable that of the very large 

 number collected nearly all are of the form now described and referred to 

 Arsinoitherium on the following grounds : — (1) their large size and massive 

 structure; (2) the agreement of their articular surfaces with those of the tibia 

 above described and with the most common form of calcaneum; (3) the presence 

 of articular surfaces for both cuboid and navicular excluding the possibility of this 

 type of astragalus belonging to PalcBomastodon, of which, moreover, the calcaneum 

 and probably the astragalus are known. 



From these considerations it will be seen that the astragalus now described may 

 be regarded with confidence as that of Arsinoitherium. The proximal articulation 

 for the tibia (text-fig. 32, tih.) is gently convex from before backwards, and very 

 slightly concave from side to side. Posteriorly there is a notch (%.), but there is 

 no perforation as in some of the Dinocerata. The articulation does not extend 

 farther back on the inner than on the outer side. On the outer side is the fibular 

 surface {fii.) looking outwards and somewhat upwards, and passing above into the 

 tibial surface. 



On the inner side the main tibial surface is continuous with an inner face looking 

 inwards and upwards, and supplying a surface for the articulation of the internal 

 malleolar process of the tibia. Beneath and behind this surface the postero-internal 

 angle of the bone forms a prominent tuberosity. The surfaces for the calcaneum 



