HISTORY OF THE EDENTATA 607 



enough has been learned of the others to show that there was 

 far less difference between the families than had arisen in the 

 later epochs. This backward convergence of the three groups 

 towards a common term plainly indicates their common origin, 

 being exactly what might have been predicted in advance of 

 experience. 



In all the genera the teeth number f ; the teeth on each side 

 were sometimes in continuous series, sometimes the first one 

 was isolated and almost always more or less tusk-like, most so 

 in "fEucholoeops. The other teeth were usually of transversely 

 elliptical shape and worn into two ridges, with a hollow be- 

 tween ; the fmylodonts {^Nematherium, etc.) already had the 

 triangular, or lozenge-shaped, lobate form of teeth, characteris- 

 tic of the family. 



The skull varied considerably in its proportions ; generally, 

 it was long and narrow, with shortened face and elongate cra- 

 nium ; the sagittal crest was seldom present, never prominent, 

 and the orbit was always widely open behind, without post- 

 orbital processes. The premaxillaries were always short and 

 toothless and in most of the genera they were slender rods, in 

 others {e.g. ^HijperUptus) broad and plate-like. The lower 

 jaw had an elongate spout-like symphysis, in which the two 

 halves were coossified, tapering forward to a blunt point and, 

 though the length of this spout differed greatly in the various 

 genera, in none was there a broad, abrupt chin such as ]Mylo- 

 don and \Megalonyx had. In \Prepotherium, which is be- 

 lieved to be referable to the fMegatheriidse, the lower jaw had 

 the extremely convex inferior border, in less exaggerated degree, 

 of its huge Pampean successor ; it would be premature to say 

 descendant. 



While the long, slender skull was the prevailing type among 

 the Santa Cruz fGravigrada, there was a group of small ani- 

 mals in which the skull was shorter and more rounded and had 

 a very suggestive likeness to that of the modern tree-sloths, 

 as was likewise true of the teeth. 



