162 University of California Publications in Geology [Vol. 10 



the bulla is incomplete and is a cavity opening below by a mouth 

 30 mm. wide. ' ' 41 



Of other skull characters considered by Hay as distinctive of A 7 . 

 texanum mention may be made of the dorso-ventral diameter or height 

 of muzzle and combined width of the nasals. In contrast to no. 15 

 additional skulls now contained in the collections of the Museum of 

 History, Science and Art of Los Angeles approach the Texas specimen 

 much more closely in height of muzzle. This is particularly true of 

 no. 166 in which the height is 49.2 mm. The combined width of the 

 nasals in the latter skull corresponds almost exactly with that in 

 A 7 , texanum. 



EELATIONSHIPS OF GENUS 



In this paper it is proposed only to touch upon some of the most 

 significant relationships of N othrotherium as brought out in a study 

 of the skull and dentition. The skeletal elements, associated with the 

 skull material in the Museum of History, Science and Art of Los 

 Angeles, furnish also a wealth of evidence concerning relation to 

 Miocene and Pleistocene ground-sloths, and until the whole skeleton 

 can be fully studied it is perhaps premature to enter on a final con- 

 sideration of the systematic position of the genus. 



The many resemblances between N othrotherium and Hapalops, as 

 shown by the foregoing comparisons, strongly support the view that 

 the former genus is derived from Hapalops as held by Scott. More 

 recently F. Ameghino 4 - in a discussion of the Miocene relatives of 

 N othrotherium derives the genus from a Hapalops-like form. It is 

 not clear which genus he considers closest in this Miocene ancestral 

 relationship, 43 for in one phyletic scheme he derives A 'othrotherium 

 from Trematherium while in another he considers the genus as descend- 

 ing from Xyophorus. According to Scott, Trematherium is very 

 imperfectly known, while Xyophorus is synonymous with Hapalops. 



As already noted, the type of skull characteristic of N othrotherium 

 has deviated less from the more generalized type of the Miocene 

 Megalonychidae than has Megalonyx. The differences between 

 N othrotherium. and Megalonyx, which in many respects make them 



4i Hay, O. P., ibid., p. 119, 1916. 



4- Ameghino, F., Notas sobre una pequeria coleccion de huesos de mamiferos 

 proeedentes de las grutas calcareas de Iporanga en el Estado de Sao Paulo- 

 Brazil, Eevista do Museu Paulista, vol. 7, pp. 99-119, 1907. 



*s Ameghino, F., ibid., pp. 105 and 119. 



