228 



About five inches from the distal end of the tibia this outer ridge be- 

 comes flattened by being, as it were, pressed against the tibia, and the 

 anterior and posterior edges are raised above the level of the tibia : 

 beyond this part the limits of the fibula begin again to be defined by 

 deep vascular grooves. The outer side of the distal end of the fibula 

 is excavated by a broad tendinous groove. The fibula and tibia are 

 distinct bones in both the Palaeotheres and Anoplotheres, as in the ex- 

 isting Pachyderms. 



950. The right astragalus of the Macrauchenia patachonica. It is with the 



Pachyderms having three toes to the hind-foot that the Macrauchenia 

 agrees in the most important characters of the present instructive bone ; 

 its anterior or scaphoidal surface, for example, is simple and not divided 

 into two equal or subecpual facets by a vertical ridge, as in the isodactyle 

 Pachyderms : and it is with the astragalus of the Tapir and Palseothere 

 that it presents the closest correspondence in the general form and minor 

 details of structure. 



If the upper or tibial articular surface be compared with that in the 

 Palceotherium magnum, it will be seen that the general direction of that 

 surface is more parallel with the axis of the bone in Macrauchenia. In 

 the Palaeotherium it is turned a little towards the outer or fibular side, 

 and in the Tapir the general direction of the same surface is placed still 

 more obliquely. The anterior border of this articulating surface is broken 

 by a semicircular notch in the Palaeothere : in the Tapir it describes a 

 gentle concave curve, and the Macrauchenia resembles the Tapir in this 

 respect. The chief difference between the astragalus of the Tapir and 

 the Palaeothere, when viewed from above, obtains in the relative length 

 of the bone anterior to the tibial articulating surface : the Macrauchenia 

 presents in this respect an intermediate structure, but differs from both 

 in the greater extent of the tibial side of this part of the astragalus. 



From the tertiary deposits of Port St. Julian, Patagonia. 



Presented by Charles Darwin, Esq., F.R.S. 



951. A metatarsal bone of the Macrauchenia patachonica. 



From the tertiary deposits of Port St. Julian, Patagonia. 



Presented by Charles Darwin, Esq., F.R.S* 



