SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 121 



the posterior opening of which is small, while the anterior opening is 

 much larger. The only other foramina present are those in the neural 

 arch for the spinal nerves. 



Compared with the atlas of Theosodon, that of Macraitchenia shows the 

 following differences : ( i ) As a whole, the atlas of the Pampean genus is 

 shorter, broader and more depressed, and the neural canal is much smaller, 

 while the neural spine and hypapophysis are much better developed. (2) 

 The anterior cotyles are more closely approximated, both dorsally and 

 ventrally, and their mesial ventral portions emarginate the inferior arch in 

 a characteristic way, not seen in Theosodon. (3) The transverse pro- 

 cesses are much longer, thicker dorso-ventrally, and decidedly recurved, pro- 

 jecting behind the posterior cotyles ; anteriorly, the processes extend the 

 full length of the atlas, converting the atlanteo-diapophysial notches into 

 foramina for the inferior branches of the spinal nerves. (4) The trans- 

 verse processes are not perforated by the canals for the vertebral arteries. 



The axis (PI. XVIII, figs. 3, 3^) is extremely elongate. Anteriorly, 

 the centrum is much broadened by the articular surfaces for the atlas, 

 which are slightly convex in both directions and are somewhat inclined, 

 sloping dorsally and backward, so that when the atlas and axis are put 

 together, the two bones form a distinct angle. Dorso-ventrally, the dia- 

 meter of the cotyles is greater than transversely. Deep sulci separate the 

 cotyles from the odontoid and, internal to each one, is a conspicuous vas- 

 cular foramen. The odontoid process is moderately elongate, peg-like 

 and somewhat depressed, with a facet for the atlas on the ventral side. 



It is very exceptional to find an ungulate animal with very long neck, 

 in which the odontoid process retains its primitive, peg-like shape. In 

 all other known instances, except the present family, the long neck is 

 associated with a spout-shape of the odontoid, and in many groups, such 

 as the horses and camels, the transformation of the odontoid keeps pace 

 with the elongation of the neck. This is another example of the curiously 

 primitive features which, in the Litopterna, so frequently persist despite a 

 high degree of specialization. 



The centrum of the axis narrows much to the middle, expanding again 

 to the posterior face, which is decidedly opisthocoelous. On the ventral 

 side of the centrum is a keel, which divides posteriorly into two quite 

 prominent ridges. The neural arch is high and narrow in front, wider 

 behind ; the spine is long antero-posteriorly, rather low and plate-like, 



