336 R. S. Lull — Pleistocene Ground Sloth, 



of the jaw. The grinding surface of the two anterior lobes is 

 directed obliquely forwards, and that of the posterointernal 

 lobe obliquely backwards, for contact with the second and 

 third superior molars. 



" The third inferior molar exhibits externally the outlines of 

 a parallelogram with rounded angles, in its cross-section, as in the 

 other species, but the inner extremity is different. The inter- 

 nal border is oblique, and not parallel to the external border, 

 owing to the fact that the anterior internal angle is much exte- 

 rior in position to the posterior internal angle, from which it 

 is separated by a shallow concavity which presents forward's 

 and inwards, as the section of a groove of the shaft, which is 

 wanting in the two larger species. The anterior and posterior 

 faces present each an open shallow median groove. 



" The last inferior molar does not differ in form from that of 

 the M. harlanii and M. robustusP 



Mylodon sidcidens Cope. 



This species was described by Cope* at the same time as the 

 preceding one, as follows : 



" This species is represented by a penultimate inferior molar 

 of a species of the size of the M. harlanii. Accompanying the 

 remains of the two species are two penultimate superior molars 

 of opposite sides, which I suspect to belong to the M. sidcidens, 

 as they differ from the corresponding teeth of the M. harlanii 

 in the same way as the penultimate inferior molar. 



"The penultimate inferior molar resembles closely that of 

 the 3f. renidens, but has the dimensions of the M. harlanii. 

 The internal extremity of the crown is beveled on the poste- 

 rior border, so that an obtuse ridge characterizes the posterior 

 side of the crown, which is separated from the posterior border 

 of the external face. The latter is subtruncate. 



" The penultimate superior molars look as though they be- 

 longed to the same individual, they so exactly agree. Their 

 long diameter is obliquely transverse to the maxillary bone, 

 and the internal face is deeply grooved as in other species of 

 the genus, the anterior ridge being much less prominent than 

 the posterior. The character which distinguishes the tooth 

 from that of the other species is that the external aspect instead 

 of being regularly rounded is obliquely beveled posteriorly so 

 as to produce an obtuse angle on the posterior side within 

 the posterior margin. This is separated from the posterior 

 internal rib by a shallow concavity. When the tooth is held 

 obliquely, as it was in the jaw, the external bevel is parallel 

 with a line connecting the two internal ribs or borders." 



*Cope, E. D., op. cit., p. 462, pi. 10, fig. 4, pi. 11, fig. 7. 



