33^ PATAGONIAN EXPEDITIONS: PAL/EONTOLOGY. 



with the principal axis of the rib ; the head bears a single circular and 

 nearly plane facet which presents backward and articulates with the first 

 thoracic ; apparently there was no articulation with the seventh cervical ; 

 the tubercle rises prominently above the neck, bearing a large saddle- 

 shaped facet. Distally, the rib is expanded and rugose and has a large 

 facet for the manubrium ; though no suture is apparent, this portion is 

 doubtless the coossified sternal rib. What is believed to be the second 

 rib has an even longer and more slender neck than the first ; the facets on 

 the head for the first and second thoracics are nowhere in contact, but 

 separated by a narrow sulcus ; the tubercle is smaller and lower than on 

 the first ; the shaft is the slenderest of all the ribs preserved, but, as in the 

 first, the principal diameter is transverse. However, this rib forms the 

 transition from the first to the succeeding ones, in which the antero-pos- 

 terior diameter greatly exceeds the transverse and which are very broad 

 and plate-like. In those ribs which articulate with two vertebrae the two 

 facets of the head are entirely separate ; passing backward, we find that the 

 anterior facet is steadily reduced, until it disappears altogether, while the 

 posterior is increased, finally covering the head. Throughout the series 

 the neck is long, though broader and more depressed than in the first and 

 second, and curving uninterruptedly into the shaft, not forming an angle 

 with it. In the "true ribs " the distal end is somewhat expanded in both 

 dimensions and bears a large, convex facet for the sternal rib, while in the 

 "false ribs" the distal end is a very rough surface. 



A single distal end of a sternal rib is preserved, which fits accurately 

 the second and third segments of the mesosternum and bears a curiously 

 close resemblance to the proximal end of a vertebral rib. The fragment 

 is, in general, very similar to the corresponding bone of Mylodon, yet differs 

 in a number of particulars ; it is more compressed antero-posteriorly, thicker 

 dorso-ventrally ; the facets on the " head " for the ventral part of the sternal 

 segments are separate, while the much smaller facets for the body of the 

 sternal segments are confluent. 



Of the sternum have been found two mesosternal segments (Plate LX, 

 figs, 7, 7") which are of exactly the same type as in Hapalops and agree 

 quite well with the second and third segments of Mylodon. It would be 

 superfluous to give a description of these bones, which differ from those of 

 Hapalops merely in size. 



