ON THE EXTINCT MAMMALS OF AUSTRALIA. 235 



side for the reception of the incurved malleolus of the tibia, and in both tiie 

 Ornithorhynchus and Echidna the tibial surface is more convex than in the 

 present fossil. 



Amongst the existing Marsupialia, the astragalus in the largest herbivorous 

 species, viz. the Kangaroo, offers very great differences from the present Au- 

 stralian fossil ; the broad and shallow trochlea for the tibia is continued upon 

 the inner side of the bone into a cavity which receives the internal malleolus, 

 whilst the fibular facet is long and narrow, and situated almost vertically 

 upon the outer side of the bone. The scaphoidal surface is unusually small, 

 convex only in the vertical direction, and divided by a vertical ridge into two 

 surfaces, the outer one being applied to the os calcis. The inferior and proper 

 calcaneal articulation is divided into two small distinct surfaces, the outer one 

 concave the inner one concavo-convex. 



Amongst the pedimanous and gradatorial marsupials, and more especially 

 in the Wombat, we at length find a form of astragalus which repeats most 

 closely the characters of the extraordinary fossil under consideration; in the 

 astragalus of the Wombat the fibular facet, of a subtriangular form, almost 

 as broad as it is long, slightly slopes at a very open angle from the ridge 

 which divides it from the tibial surface ; this surface, gently concave from 

 side to side, and more gently convex from behind forwards, I'epeats the more 

 striking character of being directly continued by its inner and anterior angle 

 with the large and transversely extended convexity for the os scaphoides. 

 The calcaneal surface below is single and continued uninterruptedly from the 

 back to the fore-part of the outer half of the under surface, and its outermost 

 part is produced into an angle, which is received into a depression at the 

 outer side of the upper articular surface of the calcaneum. Thus all the 

 essential characters of the fossil are repeated in the astragalus of the Wombat. 

 The differences are of minor import, but are sufficiently recognizable ; in the 

 Wombat, for instance, the single calcaneal surface is directly continued into 

 the cuboido-scaphoidal convexity instead of being separated from it, by a 

 narrow rough tract, as in the fossil ; the calcaneal surface is also narrower 

 than in the fossil, and the outer angle is less produced : the division of the 

 tibial trochlea for the inner malleolus is better defined in the Wombat, and 

 the depression, round which sweeps the continuous smooth surface between 

 the tibial and scaphoid surfaces, is less deep in the Wombat ; the scaphoidal 

 convexity is also less developed in the vertical direction in the Wombat. 



We thus find that the great fossil astragalus from Australia, viewed in 

 reference to the general characters of that bone in the Mammalian class, offers 

 remarkable peculiarities ; and we further find that these are exclusively and 

 very closely repeated in certain Australian genera of Marsupialia, and espe- 

 cially in the bulkiest of the existing vegetable feeders which are not saltatorial. 

 The inference can hardly be resisted, that the rest of the essential peculiari- 

 ties of the marsupial organization were likewise present in that still more 

 bulky quadruped of which the fossil under consideration once formed part. 



In the Kangaroo and the smaller leaping Marsupials the fibula is dispro- 

 portionately slender and immoveably attached or anchylosed to the tibia, 

 reminding one of the Ruminant type of organization ; it sustains little of the 

 superincumbent weight, and has no resting-place upon the astragalus, the 

 outer malleolus being simply applied to the vertical outer surface of that 

 bone. The broad and nearly horizontal surface in the present fossil clearly 

 bespeaks the existence in the same animal of a fibula which must have almost 

 equalled the tibia in size at its distal end, and have taken as large a share in 

 the formation of the ankle-joint as it does in the Wombat. We may in like 

 manner infer that the tibia and fibula were similarly connected together, and, 



