A NEW MYLODON. 333 



Of the remaining sternebra there are five preserved, four of which are appar- 

 ently consecutive and at the posterior half of the series. These are more or less 

 squarish in dorsal view, with a demifacet at each corner ventrally, the two larger 

 at the anterior corners. As in M. robustus the articulation of each sternal rib 

 except the first is by means of these demifacets, and two others borne by a 

 median ventral keel on each sternebrum. In two of the pieces this keel is nearly 

 as long as the sternebrum, in another it is much shorter. The fifth sternebrum 

 has no demifacets at the posterior corners, and so was probably the last to articu- 

 late with the ribs. The hinder part of its keel is lost. This piece differs further 

 in its narrowness and greater proportional length. It articulated with yet an- 

 other posterior piece, which seems to be lost. In M. robustus the last piece of 

 the sternum is sunilar to the one here believed to be the penultimate segment. 

 Following are the measurements of these five pieces : — 



Median length of dorsal surface 



Greatest anterior width across demifacets 



Least transverse width (near middle) 



Greatest posterior width 



Greatest length of ventral keel 



Greatest depth (tip of keel to dorsal border of sternebrum) 



In a mounted skeleton of M. robustus in the Museum, there are eight sterne- 

 bra. 



But five of the sternal ribs are entire, and these seem to be of the same type 

 as in M. robustus. Two of the smaller measure 157 and 146 mm. respectively 

 in greatest length. Distally there are two oval facets, their tips meeting, which 

 articulate with the facets on the keels of two consecutive sternebra. A short 

 distance proximal to these is a single rounded facet with more or less indication 

 of two faces for articulation with the demifacets at the dorsal corners of the 

 sternebra. The" largest sternal rib preserved is 282 mm. long, and differs in 

 having a single large terminal facet instead of two. It has also an oval elevated 

 rugosity 38 X 18 mm., at the first third of its length dorsally, which probably 

 served for muscle attachment. In the middle portion of their length these sternal 

 ribs show a T-shaped or Y-shaped cross section, the stem of which forms a ^-entral 

 keel. Two other small sternal ribs are flattened and without the keel. In one 

 of these the terminal facets for four articulations are continuous, forming a closed 

 ring. The distal portions of those ribs that did not articulate with the sternum, 

 ended in tapering bony points, one of which is preserved. 



