63 



of the greatest interest, since they show that the giant of the present extinct 

 family had the same complicated interlocking of the sternal ribs with the sternum 

 as in its less bulky congener. The manubrium sterni of the Megatherium is 

 relatively broader than in the Mylodon, except where it joins the sternum, and 

 there it is narrower. The lateral borders above the costal articulations are con- 

 vex ; these articular surfaces are triangular, and relatively broader and shorter 

 than in the Mylodon ; their posterior interspace is less concave and is rough : 

 the articular surface for the succeeding sternal bone is oval, convex, and slopes 

 obliquely from behind forwards and downwards ; there are no surfaces for the 

 second pair of sternal ribs. 



The bone of the body of the sternum, described in Mr. Cliffs Memoir on the 

 Megatherium * as having ten articular surfaces, appears to have come from near 

 the posterior end of the series. It consists, as in the Mylodon, of two parts, an 

 anterior and a posterior, but these are of similar shape and nearly equal dimensions. 

 The posterior division has the four angles truncated for the costal articulations, 

 and has two larger semicircular articular surfaces, one at the anterior and another 

 at the posterior ends, for the adjoining sternal bones. The anterior division has 

 merely the four surfaces for the anterior or terminal articulations of the bifurcated 

 sternal ribs ; which surfaces are elhptical, and are larger and deeper than those 

 on the posterior division of the bone. This bone clearly indicates the same 

 essentially complex junction of the costal arches with their sternal keystones, 

 which characterizes the Mylodon, the minor modifications being adapted to the 

 specific peculiarities of the more bulky species. It is probable also that the 

 internal division may be broader in the more anterior sternal bones of the 

 Megatherium. 



In one of the sternal bones of the Megatherium, described by Mr. Clift, only 

 the anterior surface presents the articular surface for the adjoining sternal bone, 

 the posterior surface being deeply excavated by a large elliptical costal surface 

 on each side. It thus appears, as Mr. Clift remarks, to be the last bone of the 

 sternum ; but some of the sternal bones are similarly disjoined in the Sloths, 

 and are articulated only to the simple extremities of the sternal ribs. The mo- 

 dification of the supposed eighth sternal bone in the Mylodon, and the close 



* Geol. Trans., Second Series, vol. iii. p. 445. 



