234 REPORT--1844. 



one-third smaller than that for the tibia. The tibial articular surface is not 

 continued upon the inner side of the astragalus, but its anterior and internal 

 angle, which becomes convex in every direction, is immediately continued 

 into the anterior scaphoidal convexity, which sweeps round a deep and rough 

 depression, dividing the outer and anterior part of the tibial trochlea from 

 the corresponding half of the scaphoidal convexity ; this has the greatest 

 vertical extent at its inner part, where it is separated by a narrow, rough 

 transverse channel from the part which rested upon the os calcis. The cal- 

 caneal surface is single, and covers almost the whole of the under part of the 

 astragalus ; the greatest proportion of it is flat and reniform, an angular tube- 

 rosity or process being continued from the concave margin, where the pelvis 

 of the kidney, to pursue the comparison, would be situated. This process 

 must have been received into a corresponding depression at the outer part of 

 the articular surface upon the calcaneum. On the inner margin of the flat 

 calcaneal surface, opposite the tuberosity, a small triangular flattened surface 

 is continued upwards upon the inner and posterior side of the astragalus, and 

 nearly touches the inner and posterior angle of the tibial trochlea. 



The length of this fossil astragalus is four inches eight lines, its breadth is 

 three inches five lines, its depth (at the base of the scaphoidal convexity) is 

 two inches and a half. 



We look in vain amongst the Pachyderms, with astragali of corresponding 

 dimensions, for the uniform and prominent convexity of the anterior articu- 

 lation, for its continuation with the tibial trochlea, and for the single and un- 

 interrupted calcaneal tract on the lower surface of the bone. The Probosci- 

 dians, which approach nearest the present fossil in the depressed form of the 

 astragalus and the flattening of the calcaneal articulation, have that articula- 

 tion divided into two surfaces by a deep and rough groove ; the scaphoidal 

 surface is likewise similarly divided from the tibial trochlea ; and no Pachy- 

 derm has the upper articular surface of the astragalus traversed by an antero- 

 posterior or longitudinal ridge, dividing it from an almost horizontal facet for 

 the support of the end of the fibula. 



The peculiar form of the astragalus in the Ruminants, and especially the 

 trochlear character of the anterior scapho-cuboidal surface, place it beyond 

 the pale of comparison. In all the placental Carnivora the scaphoidal con- 

 vexity is pretty uniform, and occupies the anterior extremity of the astragalus, 

 as in Man and Quadruraana; but it is more produced in the Carnivora and 

 supported on a longer neck, which is also more oblique than in the Quadru- 

 mana, where the astragalus already begins to recede in this character from the 

 Human type. In the Seals the upper surface of the astragalus somewhat resem- 

 bles the present fossil in the meeting of the tibial and fibular facets at an obtuse 

 angle formed by a longitudinal rising, but the fibular surface is rather the wider 

 of the two, and the tibial one is divided by a broad rough tract from the sca- 

 phoidal prominence ; but in addition to this anterior production of the bone 

 there is also another process from its posterior part, which, as Cuvier remarks, 

 gives the astragalus of the Seal the aspect of a calcaneum. In some of the 

 remarkable peculiarities which the astragalus presents in the order Briita it 

 approaclies tlie Australian fossil under consideration : in the Mylodon, for 

 example, where the surface for the calcaneum is single and undivided. But 

 in this great extinct leaf-eating quadruped the calcaneal facet is continued 

 into the navicular facet, which, on the other hand, is separated by a rough 

 tract from the tibial articulation, as in all the Edentata, recent and fossil. The 

 latter character likewise distinguishes the astragalus of the Rodentia from the 

 fossil astragalus under consideration. 



In the Ornithorhynchus the astragalus has a deep depression on its inner 



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