* 
302 A NEW MYLODON. 
articular facets. The ascending processes on which these are borne at once 
become more rod-like and diminish in size to mere protuberances at the anterior 
edge of the centrum, yet still distinguishable on the penultimate vertebra. 
In the 7th vertebra from the end, the roof of the neural canal becomes open. 
The terminal vertebra is a rounded knob, 24 * 20 mm. in transverse and vertical 
diameters, hollowed slightly on its anterior face and thickened ventrally. A 
slight median emargination marks its dorsal side. All but the last three caudals 
have articular facets for the chevron bones. The fourth from the end has fused 
with the chevron between it and the next anterior vertebra. All the seven 
chevrons preserved are slightly larger on one side (usually the left) than on the 
other, and are deeper anteriorly. 
Ribs.— Unfortunately the ribs are broken into many fragments and these 
are preserved in part only. From a study of the vertebrae it seems clear that 
there were sixteen pairs that had articulation by the head and the tubercle as 
shown by the facets on those bones. In Mylodon robustus there were also six- 
teen pairs each with double articulation. 
A portion of the first rib is preserved, showing it to have been thin but 
broad, rather short and curved. What were probably succeeding ribs, are repre- 
sented by thin flattened pieces. The stoutest ribs of all were over 50 mm. 
broad at the upper portion with the external outline irregularly ridged. The 
articulating facets are nearly or quite flush with the general surface of the sur- 
roundng bone, not raised as tubercles or constricted off. A depression of vary- 
ing size is present internal to the tubercular facet. 
Sternum.— A large portion of the manubrium and five of the sternebra 
are preserved. The former seems to have been composed of two pairs of elements, 
thoroughly fused, as indicated by a transverse suture still traceable separating 
the anterior from the posterior pair. There is a median round perforation ex- 
tending quite through the anterior portion of the manubrium, indicating the 
original separation of the two lateral elements. Compared with that of Mylodon 
robustus, the manubrium is much shorter and proportionately broader; the 
greatest width was apparently about the same as the length, but owing to the 
loss of parts of the periphery, exact measurements cannot be given. As in the 
former species one pair of sternal ribs seems to have articulated wholly with the 
manubrium on lateral protuberances borne at the widest portion and at about 
one half its length, or just in advance of the second pair of fused sternal elements. 
Posterior to this is a sudden contraction marking the second portion of the 
manubrium, whose demifacets for articulation with the second pair of sternal ribs 
are lost. 
