436 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



half of a mandible (No. 15,066), collected by Mr. Peterson from the Upper 

 Santa Cruz beds at Killik Aike. Owing to the small amount of material 

 available, it has not been possible to distinguish between generic and 

 specific characters. The principal dependence for the correctness of the 

 identification has, consequently, been upon size. The dimensions given 

 below agree closely with Ameghino's figures for Callomenus ligatus. 



MEASUREMENTS. 



Length, posterior border of median incisor to M T inclusive . . . .0155 



" of space occupied by vestigial teeth and posterior premolar . . -0055 



" " molar series on alveolar border. . . . . . . .010 



Width of base of incisor .......... .0015 



Depth 0025 



Posterior premolar, antero-posterior diameter ...... .0015 



" " transverse " ...... .001 



M T , antero-posterior diameter ......... .0045 



" transverse " ......... .002 



M^-, antero-posterior " ......... .003 



" transverse " ......... .002 



MJ-, antero-posterior " ......... .002 



" transverse " ......... .0015 



M-J-, antero-posterior " ......... .0013 



" transverse " ......... .001 



Depth of mandible below last vestigial tooth ...... .0043 



" " " " posterior premolar ...... .0045 



" " M T 005 



" " M T 0055 



DECASTIS Ameghino. 



(Plate LXIV, Figs. 4, 4, 6, 6a.) 



Decastis Amegh. ; Nuevos Restos Mamif. F6s. Patagonia Austral, p. 19, 



Aug., 1891 ; Revista Argentina, I, entr. 5*7, p. 305, Oct., 1891. 

 In Decastis, premolar reduction is carried one step farther than in 

 Callomenus. The heel of the posterior premolar is still retained, but the 

 whole tooth is smaller and no longer double-rooted. Between the last 

 premolar and the median incisor are four vestigial single-rooted teeth. 

 The base of the incisor is retained in one specimen (No. 9594 American 

 Museum, PI. LXIV, figs. 4, 40), showing a large pulp canal, but broken 

 off too far below the crown to indicate the distribution of the enamel. 

 The tooth is larger than in Callomenus, but similar in shape at the base. 



