FOSSIL MAMMALIA. 51 



therefore, to the consideration of the inferior articulating surface of the bones of 

 the leg (fig. 4, PI. XIII.) 



Since, of the hind foot, we possess in the present collection only a single tarsal 

 and metatarsal bone, the structure of the distal articular surface of the tibia is 

 attended with peculiar interest, because we are taught by Cuvier that it reveals to 

 us in the Ungulate animals the didactyle or tridactyle structure of the foot. In the 

 Ruminants this articular surface is nearly square, and extended transversely be- 

 tween two perpendicular malleoli, while in the Pachyderms with three toes to the 

 hind-foot the articular surface of the tibia is oblique, and is divided into two 

 facets between the perpendicular malleolar boundaries. Now in the Macrauchenia, 

 although the two bones of the leg are anchylosed together, the extent of that part 

 of the tarsal articular surface which is due to the tibia is indicated, as in the case 

 of the radius in the joint of the fore-arm, by a groove ; and we are thus able to 

 compare this surface with the distal articular surface of the tibia in the Palseo- 

 there and Anoplothere. It presents in the Macrauchenia a very close resem- 

 blance with that of the Palceotherium magnum* being divided into two facets by 

 a convex rising, which traverses the joint from behind forwards ; but the ridge 

 is narrower, the internal facet somewhat deeper, and the external oblique surface 

 rather flatter than in the three-toed Palseothere. In the portion of the tarsal 

 articular surface due to the fibula, we find, however, a more marked deviation from 

 the Palseothere, and an interesting correspondence with the Anoplothere, in the 

 inferior truncation and horizontal articular surface which is continued upon the 

 lower extremity of the fibula, at right angles with the vertical malleolar facet 

 which forms the outer boundary of the trochlea of the astragalus : this articular 

 surface unerringly indicates a corresponding articular projection in the calcaneum, 

 which, therefore, although the bone itself does not form part of the present collec • 

 tion, we may conclude to differ from the calcaneum of the Palgeotherium, and to 

 resemble that of the Anoplotherium, in this particular at least. 



The valuable indication which the distal articular surfaces of the anchylosed 

 tibia and fibula have given of the correspondence of the hind-foot with the fore- 

 foot of the Macrauchenia, in regard to the number of the toes, receives ample 

 confirmation from the astragalus, which, of all the bones in the foot, is the one 

 that an anatomist would have chosen, had his choice been so limited, and which 

 most fortunately has been secured by Mr. Darwin, in a very perfect state, in the 

 present instance. I have compared this astragalus with that of the Giraffe, and 

 other Ruminants, the Camel, the Anoplothere, the Horse, the Hog, the Hippopota- 

 mus, Rhinoceros, Tapir, and Palaeothere : it is with the Pachyderms having three 

 toes to the hind-foot, that the Macrauchenia agrees in the main distinguishing 



* See Ossein. Foss. iii. PI. XXVI. fig. 5. 



