164 University of California Publications in Geology [Vol. 10 



in the asphalt beds in greater numbers than Megalonyx is singular, 

 considering the apparently more restricted range of the former genus 

 in North America. There is reason for believing, however, that 

 Nothrotherium nourished in California and Texas during a portion 

 of the Pleistocene. 



The species of Nothrotherium from the asphalt beds, originally 

 considered by the writer as new, is now held to be identical with N. 

 shastense Sinclair from Potter Creek Cave. Further investigation 

 may justify a subspecific separation of the Rancho La Brea form, but 

 at present there is not sufficient reason for a specific distinction from 

 the Potter Creek Cave species. 



The many characters held in common by A r . texanum and N. 

 shastense from Potter Creek Cave and Rancho La Brea suggest specific 

 identity of the two forms. 



Nothrotherium is very closely related to the Miocene genus 

 Hapalops. Comparison between the two genera presents nothing 

 which can be considered adverse to the view held by Scott, that 

 Nothrotherium is the direct descendant from Hapalops. The Pleisto- 

 cene genus shows several noticeable advances beyond the Miocene 

 form in skull and dentition. The structure of the skull in Nothro- 

 therium indicates a type which has deviated less than Megalonyx 

 from a generalized skull characteristic of the Santa Cruz ground-sloths. 



The great dissimilarity between Nothrotherium and Megalonyx in 

 many characters of skull and dentition substantiates the views of Scott 

 and Ameghino that these genera are the results of two divergent lines 

 of evolution already separate in the Santa Cruz Miocene. The differ- 

 ences between the two Pleistocene genera are in many instances so 

 great as to suggest a separation of subfamilies rather than genera. A 

 close resemblance between the two forms is shown by the position and 

 general shape of the molariform teeth, in which characters both genera 

 are more closely related to each other than either is to Megatherium. 



Transmitted June 16, 1916. 



