CEPHALOMYS PLEXUS 



I 9 I 



Measurements 



Upper dentition, pm. 4 to m. 3 

 Upper dentition, pm. 4, length 

 Upper dentition, each molar length 

 Upper dentition, each molar width 



Specimen 



3091 

 9. mm. 



2.75 mm. 

 2 . 1 mm. 

 2.25 mm. 



Ameghino's 



Type 



9.5 mm. 



The lower jaw is slender, the incisor being relatively both 

 smaller and slenderer than in C. arcidens; the back part 

 of the ramus light and thin, the coronoid process being a 

 tiny spur, and the articular condyle of small size, and on a 

 level with the teeth. 



Measurements 



Lower dentition, pm. 4 to m. 3 

 Lower dentition, in. 1 to pm. 4 

 Lower dentition, pm. 4, length 

 Lower dentition, each molar, length 

 Lower dentition, each molar, width 

 Height of mandible under pm. 4 



B 



ext. 



Fig. 122. C. plexus, left lower premolar, 

 molar series; A, of young individual; B, 

 of old individual; int., internal side; ext., 

 external side, X4/1. 



Fig. 123. C. prosus, left upper 

 premolar, molar series, x 4/1. 



p*t.V 



m .3. 



Fig. 124. C. prosus, premolar 4 

 and molar 2, x 4/1. 



Cephalomys prosus Ameghino 



C. prosus Amegh., 1902, Bol. Acad. Nac. Ciehc. Cordoba, t. 17, p. 37. 



This is the tiniest species of the genus, and least fre- 

 quently found, probably because on account of the small 

 size it was more frequently destroyed before burial, and 

 also because it is hard to find such tiny specimens; so that 



