346 



R. S. Lull — Pleistocene Ground Sloth, 



Texas specimen. Here the hinder tooth differs from any of 

 the four jaws compared in the relatively smaller posterior 

 lobe, and in having but the slightest trace or tendency to 

 a three-lobed condition. M. harlani in this respect stands 

 at one end of a series, with the Paramylodon tooth at the 

 other. The California jaw is next to harlani, then comes the 





3d molar 



4th molar 



f 10264 

 i 

 Texan jaws<J 



i 



1 1026*7 



quatrefoil, unique 



like California type 



like Paramylodon 



like Paramylodon 



Californian jaw 



like Paramylodon 



like 10264 



larger Texan, the smaller Texan, and finally the Paramylodon, 

 the last being the most complex. 



With regard to the penultimate molars, the smaller Texair 

 jaw has the simplest, M. harlani next, the California jaw 

 next, with the tooth of the larger Texan jaw the most com- 

 plex. Little can be learned from the jaw of M. harlani, as it 

 has apparently been subject to abrasion, whether in the cast or 

 in the original I do not know, which has destroyed most of the 

 surface and probably altered its contour. As it is, it resembles 

 the smaller Texan jaw in the smooth youthful roundness of 

 the curves, but in weight and proportions it is nearest the larger 

 Texan individual. 



Vertebrae. 



The vertebral formula for Mylodon robustns given in 

 Flower's Osteology is : cervicals 7, dorsals 15, lumbar-sacrals 

 10, caudals 21 ; and the mounted or figured specimens known 

 to me approximate this number. On the other hand, the 

 mounted skeleton of Mylodon from the Rancho la Brea, now 

 in the Museum of History, Science and Art in Los Angeles, 

 exhibits at least twenty-three pairs of ribs, a number largely in 

 excess of those in M. robitstus. That there is a chance of 

 error in a composite skeleton, however, must be admitted. 

 The Texas material before me cannot settle this matter, as the 

 vertebrae and ribs of at least three individuals may have been 

 present and commingled. If all the bones present were of one 

 animal, the result would be far more conservative than that of 



