166 
TERTIARY VERTEERATA OF THE FAY()M. 
is the larger and is widest from before backwards ; at its antero-external angle it runs up 
on to the prominent intercondylar process (ic.p.), on the outer side of which there is a 
deep fossa for the attachment of ligament. The outer facet (o.) is widest from side to 
side, and beneath its outer edge there is a flattened surface, apparently for the upper end 
of the fibula. The upper part of the not very prominent cnemial crest (cn.) is divided 
into two by a deep depression, the outer portion being the largest. The middle portion 
of the shaft is flattened behind and convex in front; towards the distal articulation it 
widens out and bears on its postero-internal surface a prominence (p.) forming the outer 
side of a groove, precisely similar to that seen in the tibia of P. headnelli. The distal 
articulation is also like that found in the large species ' there is a prominent internal 
malleolus (i.m.) bearing an astragalar facet looking outwards and forwards ; external to 
Text-fig. 57. 
Right ulnca of Palaomastodon {^)parvus\ A, from front; B, from inner side. 
oh, olecranon process ; rp., radial groove; r.s., radial surface. | nat. size. 
this is the main surface for the astragalus (a.s.), strongly concave from before backwards, 
and is produced downwards into a blunt ])oint both lad'ore and behind. External to this 
surface there is a roughened facet (/.) looking downwards and outwards, apparently for 
articulation with the fibula. The chief dithu'ence between this tibia and that of h'lejihas 
is in the form of the di.stal articulation. In the recent form, probably owing to the 
increasing size and weight, the astragalar surface is much broader and more gamtly 
concave, and this increase of width has led to the reduction of the internal malleolus to 
a mere blunt jjoiut. Otherwise the two tibias are very similar even in small points. 
