432 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



ACAREMYS MINUTUS Ameghino. 

 (Plate LXVII, Figs. 10-10", 13.) 



Acaremys minutus Amegh. ; Enumeracion sistem., etc. ; 1887, p. 9. 



This species is smaller and more delicate in all its proportions than A. 

 murinus and in a number of respects its skull is very characteristic. The 

 incisors are narrow and slender, with strongly convex faces, and the grind- 

 ing teeth are very small ; p- and m a are slightly smaller than m 1 and a , 

 but m a is relatively less reduced than in A. murinus. The upper incisors 

 are less strongly curved than in any of the other species of this genus, 

 or Sciamys, so that the alveoli do not extend so near to the dorsal border 

 of the premaxillaries, leaving a wider, thin and plate-like strip between the 

 alveolus and the nasal suture and giving quite a characteristic appearance 

 to the rostrum. The latter is rather slender and tapers anteriorly, though 

 not in any marked degree. The frontals are nearly plane, with prominent 

 supraorbital ridges, but with no definite postorbital processes (Fig. 10, PI. 

 LXVII, is erroneous in this respect). On the other hand, the jugal has a 

 distinct postorbital angulation ; anteriorly the jugal ends in a point, not 

 ascending the zygomatic process of the maxillary, and posteriorly it is 

 continued to the glenoid cavity. As in the other species of this genus, 

 the dorsal branch of the maxillary zygomatic process is quite slender, but 

 its broad side is turned more obliquely forward and less laterally. A 

 peculiarity of A. minutus is the shape of the posterior nares, the front 

 wall of which slopes ventrally and forward, forming a narrow, sharply 

 pointed, V-shaped depression in the palate, which extends almost to m-. 



MEASUREMENTS. 



Upper dentition, length 019 Rostrum, length 0125 



pi-mi 007 " width at base 0085 



Upper incisor, width 0015 " " " anterior end 006 



" thickness 0023 " dorso-ventral height 008 



Skull, interorbital width 0085 



An isolated mandible has the following dimensions : 



Lower dentition, length 0165 Lower incisor, width 0015 



" " p-m 008 " " thickness 002 



