126 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



responding ones of Macrauchenia, figured by Burmeister ('64, PL II, fig. 

 17), but in the Pampean genus the metapophyses are not so prominent, 

 while the neural spines are narrower, thicker and more recurved. 



In the middle of the thoracic region, in Theosodon, the neural spines 

 become narrow, more spaced apart and lower, though retaining their back- 

 ward inclination through nearly the entire region. In the hinder part, the 

 spines become shorter, broader and more erect; the anticlinal vertebra 

 may be either the last or the penultimate thoracic. The transverse pro- 

 cesses remain prominent throughout the series, gradually descending from 

 the neural arch to the centrum. The facet for the rib-tubercle persists 

 even on the last vertebra, in which the process has a different form, being 

 broader and more depressed. Between the tenth and eleventh thoracic 

 vertebrae, takes place the transformation of the zygapophyses from the 

 thoracic to the lumbar type. On the tenth, the prezygapophyses still 

 present upward and are small and nearly flat, but the metapophyses, which 

 are now close to them, are strongly concave on the mesial side and curve 

 around the postzygapophyses of the ninth, though not actually articulating 

 with them. The postzygapophyses of the tenth are convex and semicylin- 

 drical and the prezygapophyses of the eleventh are concave. 



The lumbar vertebrae (PI. XVIII, fig. 7) are sometimes five and some- 

 times six in number. Except the last one, they have centra which are 

 laterally compressed and of subtrihedral shape, much like those of the 

 thoracic vertebras, but larger, instead of the broad, depressed centra, usual 

 in the lumbar region. The last lumbar vertebra has a short, depressed 

 centrum, with broad anterior, very narrow and convex posterior face. As 

 in all of the known Santa Cruz Litopterna, the zygapophyses are of the 

 semicylindrical, interlocking character, most resembling those of the Arti- 

 odactyla. Metapophyses are present throughout, but are less prominent 

 than in the thoracic region and are very small on the penultimate lumbar. 

 The neural spines are high, broad and strongly curved forward and, except 

 in the first and second lumbars, they have not the abruptly truncated and 

 thickened free ends which occur in the lumbar region of nearly all mam- 

 mals, but are gently rounded. On the last vertebra, the spine is decidedly 

 smaller than on any of the preceding lumbars. 



The transverse processes are broad and curved forward ; this breadth 

 increases posteriorly, reaching a maximum on the last vertebra. On the 

 hinder border of each transverse process of the penultimate lumbar and 



