iy8 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



dence extends, there is much reason to believe that Prepotherium is nearly 

 allied to the ancestor of Megatherium, and that Nematherium and Analci- 

 therium were the fore-runners of Mylodon, Lestodon and Scelidotherium. 

 Finally, should be remarked another notable difference between the 

 Santa Cruz Gravigrada and those of the Pampean. In the former epoch 

 the Megalonychidce were the dominating family, which included the greater 

 number of genera and species and the overwhelming majority of individ- 

 uals, while the other two families are so rarely found that they must have 

 formed but an inconspicuous element of the fauna. In Pampean times, 

 on the contrary, most of the Megalonychidce had become extinct and the 

 Megatheriidce and Mylodontidce had greatly increased and assumed the 

 dominating position. 



MEGALONYCHIDCE. 



In the Santa Cruz epoch, at least in Patagonia, this was the characteristic 

 family of Gravigfada, far outnumbering in genera, species and individuals 

 both of the other families combined, and it is the only one of the three in 

 which the skeletal structure is at all completely understood, but enough is 

 known of the other two families to display many of their contrasting 

 characters. The present family includes many very small species, the 

 smallest of the known Santa Cruz ground-sloths, as well as some of the 

 largest, with every intermediate gradation of size. The teeth are non- 

 lobate and may be arranged in continuous series, or, as is much more 

 common, with the first tooth in each jaw isolated by diastemata and, 

 though varying much in size and shape, always more or less distinctly 

 caniniform ; - is usually implanted in the jaw near the maxillo-premax- 

 illary suture, but in a few genera is placed considerably behind that suture. 

 The molariform teeth are most frequently of transversely oval form, but 

 are often rectangular or subtrihedral. 



The skull is elongate and cylindrical, usually with deep postorbital 

 constriction and preorbital fossae. The sagittal and occipital crests vary 

 much in development and, though never very prominent, may be quite 

 distinct, very faint, or quite obsolete. The premaxillaries, which differ 

 greatly in length, are yet of a nearly uniform type ; each is composed of three 

 branches, which are almost always- slender and rod-like, but in Hyper- 

 leptus form broad, tapering plates, with minute incisive foramina. The 

 palate is very rugose and between the molariform teeth is strongly con- 

 vex. The mandible has an edentulous beak or spout, which varies remark- 



