FOSSIL MAMMALIA. 



41 



last lumbar vertebra, which presents in addition, the following characters ; each 

 transverse process sends off from its posterior margin a thickened and transversely 

 elongated protuberance, which supports a flattened articular surface adapted to a 

 corresponding surface on the anterior part of the transverse process of the first 

 sacral vertebra : it likewise presents on its anterior edge a flattened and rough 

 surface, which is closely attached by ligamentous substance to the opposite part 

 of the transverse process of the penultimate lumbar vertebra. A similar struc- 

 ture exists iu the last two lumbar vertebra? of the Rhinoceros, Tapir, and Horse. 

 In the latter animal, anchylosis of these articulating surfaces of the lumbar and 

 sacral vertebrae generally takes place with age, and, judging from the character 

 of the same surfaces in the Hippopotamus, the motion of its lumbar vertebra: 

 upon the sacrum may in like manner become ultimately arrested. 



Now in the Macrauchenia, as in the Pachyderms above cited, the transverse 

 processes of the last lumbar vertebra: are of considerable thickness and extent, 

 and are joined by enarthrosis to the transverse processes of the sacrum ; but the 

 bony structure of these joints would indicate that they were not subject to be 

 obliterated by anchylosis. The articular surfaces which project from the pos- 

 terior part of the transverse processes of the last lumbar vertebra: present a re- 

 gular and smooth concavity, adapted to a corresponding convexity in the transverse 

 processes of the first sacral vertebra. These articulating surfaces have evidently 

 been covered with smooth cartilage ; they present a pretty regular transverse 

 ellipsoid form. A view of the three joints by which, independently of the two 

 oblique processes, the last lumbar vertebra of the Macrauchenia was articulated 

 with the sacrum, is given in Plate VIII. fig. 1. The transverse processes of the 

 posterior lumbar vertebra, besides their agreement with those of the Horse and 

 Hippopotamus in the structure just described, also correspond with them in 

 general form, and deviate remarkably from those of the Camelidce in their great 

 breadth. 



It will be seen that the articulations on the body and transverse processes of 

 the last lumbar vertebra of the Macrauchenia differ from the corresponding articu- 

 lar surfaces of the Horse, inasmuch as the middle surface is convex, while the 

 two lateral ones are concave, and these are moreover relatively larger than either 

 in the Horse or Hippopotamus : by this structure the trunk was more firmly 

 locked to that segment of the vertebral column, which receives and transmits to 

 the rest of the body the motive impetus derived from the hinder extremities, 

 which are in all quadrupeds the chief powers in progression ; while at the same 

 time the shock must have been diminished by the great extent of interposed 

 elastic cartilages ; and a certain yielding or sliding motion would be allowed 

 between the lumbar vertebrae and sacrum. 



The anterior oblique processes of the lumbar vertebrae of the Macrauchenia 



