SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 437 



The last vestigial tooth (PI. LXIV, fig. 6), probably representing the 

 median premolar, has a cylindrical, blunt-pointed crown. The crown of 

 the third intermediate (PI. LXIV, fig. 4), overhangs anteriorly. The first 

 two rudimentary teeth have been shed, but from the obliquity of their 

 alveoli, it is probable that they resemble the homologous teeth in Palceo- 

 thentes. 



The molars are an exact duplication of those of Callomenus, but are 

 slightly more worn in both the mandibles in the collection. The last 

 tooth is double-rooted, but has, unfortunately, been broken off. 



The number and relative position of the mental foramina is much the 

 same as in Callomenus. The apparent shifting forward of the anterior 

 foramen is due to the reduction in length of the premolar series. The 

 masseteric fossa is imperforate and the coronoid broad and high with 

 prominent anterior margin. The symphysial impression is wider than in 

 Callomenus. 



Difference in size is the only character available for the separation of 

 the two species recognized by Ameghino (1891, p. 305), of which the 

 larger, D. columnaris, is represented in the collection by two specimens. 



DECASTIS COLUMNARIS Ameghino. 



(Plate LXIV, Figs. 4, 40, 6, 6a.) 



Decastis columnaris Amegh. ; Nuevos Restos Mamif. F6s. Patagonia Aus- 

 tral, p. 19, Aug., 1891 ; Revista Argentina, I, entr. 5, p. 305, Oct., 

 1891. 



Like Callomenus ligatus, Decastis columnaris is known only from lower 

 jaws, of which parts of two are in the collections at Princeton University 

 and the American Museum. No. 15,710, collected by Mr. Peterson at 

 Coy Inlet, is a portion of the left half of a mandible, with two premolars 

 and three molars in place (PI. LXIV, figs. 6, 6a). The second specimen 

 (No. 9594 American Museum) from the Santa Cruz beds, on the Rio 

 Gallegos, is the anterior half of a right mandibular ramus retaining in 

 place two molars, the third vestigial tooth and the base of the incisor 

 (PL LXIV, figs. 4, 4^)- 



MEASUREMENTS. 



No. 15,710. No. 



Approximate length, posterior border of median incisor to M T 



inclusive. . . . . . . . . . .016 



Length of space occupied by vestigial teeth and posterior premolar. .0055 



