

46 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



Macrauchenia the roughened surface, (&. fig. 1, PL X.) commencing also at the 

 outside, extends only one-third of the way across the articular surface : it is, 

 however, as shallow as in the Camel. The articular surface on the anterior part 

 of the base of the olecranon is broader in the Hippopotamus than in the Camel ; 

 but in the Macrauchene it is twice as broad as in the Hippopotamus. The size 

 of the olecranon in the Macrauchene exceeds that of the Hippopotamus, and a 

 fortiori that of the Camel : indeed in its general magnitude the Macrauchenia 

 must have fully equalled the largest Hippopotamus ; but it no doubt had a more 

 shapely, and less broad and bulky trunk. The olecranon of the Macrauchenia 

 differs in shape, both from that of the Camel and Hippopotamus ; it terminates 

 above in a three-sided cone with an obtuse apex ; and presents a well marked pro- 

 tuberance at the outer side of the base, which is not present in either the 

 Camel or Hippopotamus. There is also a strong rugged ridge on the back 

 part of the olecranon which makes an angle before sinking into the level of the 



ulna below. 



The confirmation of the close affinity of the Macrauchenia to the Pachyder- 

 matous Order, which the structure of the cervical vertebrae alone might have ren- 

 dered very doubtful, is afforded by the bones of the right fore-foot (PL XI.) ; 

 these are fortunately in so perfect a condition, as to make it certain that this 

 interesting quadruped had three toes on the fore-feet, and not more ; and that the 

 fully developed metacarpal bones are distinct, and correspond in number with 

 the toes, and are not anchylosed into a single cannon bone, as in the Ru- 

 minants. The bones preserved are the metacarpals, proximal phalanges, and 

 middle phalanges of each of the three toes, and the distal phalanx of the inner- 

 most toe. 



The proximal end of the innermost metacarpal bone presents three articular 

 surfaces ; the middle facet is the largest, and the two lateral ones slope away 

 from it at an angle of 45°. The middle facet is broad and slightly convex in 

 front, narrow and concave behind ; the distal articular surface of the trapezoides 

 must have corresponded with this surface; the outer facet is narrow, flat, extends 

 from the fore to the back part of the head of the bone, and must have been 

 adapted to a corresponding surface on the os magnum; the inner facet is the 

 smallest, presents a triangular form, and is situated towards the back part of the 

 head of the metacarpal bone ; it indicates the existence of a rudimental meta- 

 carpal bone, or vestige of a pollex. Below the outermost of the lateral surfaces 

 there is a crescentic articular surface with its concavity directed outwards and 

 downwards (fig. 2, PL XV.), against which a corresponding convex articular sur- 

 face of the middle metacarpal abuts, (fig. 3, PL XV.) External to this surface 

 the proximal end of the middle metacarpal bone presents two articular surfaces 

 for the carpus ; the larger one, which was adapted to the os magnum, is hori- 



