224 



937- The proximal extremities of the anchylosed radius and ulna of the Ma- 

 crauchenia patachonica. 



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



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



The portion of the antibrachium of the Macrauchenia which is pre- 

 served, presents a condition of the radius and ulna intermediate to those 

 which respectively characterise the same bones in the Pachyderms and 

 Camels. In the former the radius and ulna are separated bones, united 

 in the prone position by ligament, but so closely and firmly that the move- 

 ment of supination cannot be performed : in the ordinary Ruminants they 

 are partially joined by bony confluence, which rarely extends to the 

 proximal extremities : in the Camel and Llama the anchylosis of the radius 

 and ulna is so complete that no trace of their original separation can be 

 perceived, and the olecranon appears but as a mere process of the radius. 



In the Macrauchenia the anchylosis of the radius and ulna is also com- 

 plete, but the boundary line of the two originally distinct bones is very 

 manifest, and the proportion which each contributes to the great articu- 

 lating surface for the distal end of the humerus is readily distinguishable. 

 About a sixth part of this surface is due to the head of the radius, which 

 enters into the composition of the anterior and outer part of the articu 

 lation, and its extent is defined by a depressed line, describing a pretty 

 regular curve, with the concavity directed forwards and a little outwards. 



The following bones belong to the same right fore-foot ; they were discovered 

 in the tertiary deposits of Port St. Julian, Patagonia, and 



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



938. The inner metacarpal bone, corresponding to the second in pentadactyle 



quadrupeds, of the Macrauchenia patachonica. 



939. The middle metacarpal bone of the Macrauchenia patachonica. 



940. The outer metacarpal bone, corresponding to the fourth in pentadactyle 



quadrupeds, of the Macrauchenia patachonica. 



941 . The proximal phalanx of the inner toe of the Macrauchenia patachonica. 



