832 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 X 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 chevj'on bones. The fourth from the end has fus(>(] 

 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 jjart only. From a study of the vertebrae it seems clear tliat 

 there were sixteen pairs that had articulation by the head and the tubercle as 

 shown b}' the facets on those l)ones. 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 re])ie- 

 sented by thin flattened pieces. The stoutest ribs of all were over 50 nini. 

 broad at the upper portion with the external outline irregularly ridged. Tlie 

 articulating facets are nearly or (juite 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 tlie 

 loss of parts of the periphery, exact measurements cannot be given. As in the 

 former species one jjair of sternal ribs seems to have articulated wholly with the 

 manubrium on lateral protuberances borne at the widest j)ortion 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 witli the second pair of sternal libs 

 are lost. 



