CEPHALOMYS PLEXUS I9I 
MEASUREMENTS SPECIMEN AMEGHINO'S 
3091 TYPE 
Upper dentition, pm. 4 to m. 3 9. mm. 9.5 mm. 
Upper dentition, pm. 4, length 2.75 mm. 
Upper dentition, each molar length 2.) “mm. 
Upper dentition, each molar width 2.25 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 SPECIMEN AMEGHINO’S 
3005 TYPE 
Lower dentition, pm. 4 to m. 3 TO. “mm: 10.5 mm. 
Lower dentition, in. 1 to pm. 4 6 mm. 6.5 mm. 
Lower dentition, pm. 4, length 
Lower dentition, each molar, length 
Lower dentition, each molar, width 
Height of mandible under pm. 4 
Fig. 123. C. prosus, left upper 
premolar, molar series, x 4/I. 
Fig. 122. C. plexus, left lower premolar, pms 
molar series; A, of young individual; B, 
of old individual; znt., internal side; ext., Fig. 124. C.prosus, premolar 4 
external side, x4/T. and molar 2, x 4/1. 
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 
