EDENTATA OF THE SANTA CRUZ BEDS. l6l 



P. australis. M. anceps. 



Cranium, width at postorbital processes 36.9 40.9 



" " over zygomatic arches 80.4 76.3 



Occiput, height 28.9 32.2 



" width 47. i 40.9 



" distance between condyles 18.8 21.2 



Zygomatic arch, length 57.9 54.3 



" " " of suborbital process 33.3 37.7 



" width across descending processes 7 2 -4 70.8 



Rostrum, length 28.9 ? 2 1 .2 



" width at base 34.7 3 5.4 



GRAVIGRADA. 



Among the edentates of the Santa Cruz formation the Gravigrada are 

 incomparably the most numerous in individuals, as well as in species and 

 genera, forming indeed one of the most conspicuous and characteristic 

 elements of the fauna. In marked contrast to what we have learned 

 regarding the armadillos and glyptodonts of the epoch, the Santa Cruz 

 ground-sloths appear to have great phylogenetic significance ; and fore- 

 runners, if not the actual ancestors of almost all the great Pleistocene 

 genera of North and South America, may be observed in this fauna. 

 Whether these Santa Cruz types are or are not the ancestors sought for, 

 cannot be satisfactorily determined until more has been learned concern- 

 ing the faunas intermediate in time between the Santa Cruz and the 

 Pampean, for phylogenetic problems can be solved only by tracing the 

 successive steps of descent. For all practical purposes, however, we may 

 regard these Santa Cruz genera as the actual ancestors, for they must at 

 least represent those ancestors in all the essentials of structure. 



Three families of the Gravigrada are represented in the Santa Cruz 

 fauna, but in very unequal proportions. By far the greater number of 

 individuals, of species and of genera are members of the Megalonychidce ; 

 the Planopsidce are much less common, and the Mylodontidce are very 

 rare. In consequence of this disproportion in the representation of the 

 three families, our knowledge of the structural characteristics of each is 

 very unequal. Several genera of the Megalonychidce are very completely 

 known ; concerning only one genus of the Planopsidce have we any con- 

 siderable information, while, except for the teeth and skull, the Mylodou- 

 tidce are almost unknown. Consequently, any general statements regard- 



