436 
PATAGONIAN EXPEDITIONS : 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 My inclusive 
“ of space occupied by vestigial teeth and posterior premolar 
“ “ molar series on alveolar border. 
Width of base of incisor .... 
Depth “ “ " 
Posterior premolar, antero -posterior diameter 
“ “ transverse “ 
My, antero-posterior diameter 
“ transverse u 
My, antero-posterior “ 
“ transverse “ 
My, antero-posterior “ 
“ transverse 
My, antero-posterior 
“ transverse 
Depth of mandible below last vestigial tooth 
“ posterior premolar 
“ My . 
“ My . 
n 
4 ( 
a 
n 
a 
.0155 
•0055 
.010 
.001 5 
.0025 
.0015 
.001 
.0045 
.002 
.003 
.002 
.002 
.001 5 
.0013 
.001 
.0043 
.0045 
.005 
.0055 
DECASTIS Ameghino. 
(Plate LXIV, Figs. 4, 4 a, 6, 6a.) 
Decastis Amegh. ; Nuevos Restos Mamlf. Fos. Patagonia Austral, p. 19, 
Aug., 1891 ; Revista Argentina, I, entr. $a, 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, 4 a), 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. 
