1917 | 



Stock: Skull and Dentition of Nothratherium 



163 



very dissimilar in structure of skull, are to be considered as end- 

 products of two widely divergent lines of evolution arising from more 

 closely related forms in the Tertiary. The writer is thus entirely in 

 accord with Scott, who states with reference to the Santa Cruz ground- 

 sloths : ' ' That Hapalops is nearly related to both Nothrotherium arid 

 Megalonyx is abundantly clear from a cursory examination of the 

 skeleton, all parts of which are very similar to those of the two 

 Pleistocene genera. In fact, there is every reason to believe that 

 Nothrotherium was the direct descendant of some species of this 

 genus. ' Hi 



With a reduced dentition. Nothrotherium is not only more special- 

 ized than Hapalops, but is also more advanced than either Megalonyx or 

 Megatherium. In a comparison of the two Pleistocene megalonychids, 

 Ameghino states: "Pero en cambio. Megalonyx que tiene cinco dientes 

 en cada lado aparece como una forma considerablemente mas primitiva 

 que Nothrotherium que solo tiene cuatro. ' Hs Judging from the struc- 

 ture of the skull, the position of the molariform teeth in Nothrotherium 

 undoubtedly corresponds to that of the four posterior teeth of 

 Megalonyx. Thus a closer relationship between these genera than 

 between Nothrotherium and Megatherium is indicated by the position 

 of the molariform teeth. This resemblance extends also to the shape 

 of these teeth. 



In Notltrotherium and Megalonyx M- is smaller than M- or M- and 

 this is usually true also for Hapalops; M- as a rule can be distinguished 

 from M± by its slightly larger size; M- is compressed anteroposterior^'. 

 In the inferior molariform series the principal character held in 

 common (with slight modifications) by the three geriera is the shape 

 and position of M 5 . In Hapalops this tooth is sub-cylindrical, while 

 in Nothrotherium and Megaton yx it has essentially the same shape but 

 with the outer side flattened. Further, in the three genera the trans- 

 verse axis of M ; is somewhat oblique to the long axis of the tooth-row. 

 In Megatherium this tooth is quadrate in shape, and transverse in 

 position, thus resembling the remaining inferior teeth. 



CONCLUSIONS 



Next to Mylodou, the genus Nothrotherium is the most abundant 

 ground-sloth found at Rancho La Brea. It is represented by decidedly 

 fewer individuals than is Mylodou. The occurrence of Nothrotherium 



** Scott, W. B., op cit., p. 181, 1903. 

 > r > Ameghino, F., op. cit., p. 103. 



