CEPHALOMYS PLEXUS 



191 



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. i to pm. 4 

 Lower dentition, pm. 4, length 

 Lower dentition, each molar, length 

 Lower dentition, each molar, width 

 Height of mandible under pm. 4 



int. 



ext. 



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

 molar series; A, of young individual; B, 

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

 external side, X4/I. 



SPECIMEN AMEGHINO'S 



3005 



10 mm. 

 6 mm. 

 3-5 mm- 

 2.5 mm. 

 2 mm. 

 mm. 



TYPE 

 10.5 mm. 

 6.5 mm. 



4.8 mm. 



Fig. 123. C. prosus, left upper 

 premolar ,'_molar series, x 4/1. 



Fig. 124. C. prosus, premolar 4 

 and molar 2, x4/i. 



Cephalomys prosus Ameghino 



C. prosus Amegh., 1902, Bol. Acad. Nac. Cienc. 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 



