2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



Preorbital bar of maxillary sloping obviously forward ; root capsule 

 of upper incisor terminating in contact with outer half of an- 

 terior border of alveolus of pm^; bases of alveoli of right and 

 left pm* separate, not encroaching on floor of narial passage ; 

 pmi with only two reentrant angles on lingual side Capromys 



Preorbital bar of maxillary vertical or sloping slightly backward; 

 root capsule of upper incisor terminating above and ectad to 

 anterior half of outer border of alveolus of pm* ; bases of alveoli 

 of right and left pm* in contact, encroaching on floor of narial 

 passage ; pm^ with a small third reentrant angle on lingual 

 side Geocapromys 



REMARKS ON GEOCAPROMYS 



Skull. — The ascending branch of the maxillary dividing the orbit 

 from the antorbital foramen is vertical (G. ingrahami) or backward- 

 sloping (G. hrownii and G. thoracatus) in relation to alveolar line 

 instead of conspicuously forward-sloping as in Capromys (pi. i, figs, i 

 and 2). By this character alone any one of the three living species 

 can be distinguished from any of the four living Capromys. (I have 

 not seen a specimen of the extinct G. columbianus in which the as- 

 cending branch is preserved). The backward slope in Geocapromys 

 is never so strong as the forward slope in Capromys, but the difference 

 is obvious when the general direction of the ascending branch is com- 

 pared with the line of the alveolar margin. 



Teeth. — Root of upper incisor encapsuled in the lower half of the 

 maxillary wall of the antorbital foramen (see pi. I, fig i), the dis- 

 tance between the outer surfaces of the very obvious incisor capsules 

 of opposite sides greater than that between the outer sides of the 

 basal capsules of the opposite first molars. In Capromys the root of 

 the incisor terminates opposite the antero-inner edge of the lower 

 lip of the antorbital foramen (pi. i, fig. 2), and the transverse diam- 

 eter of the rostrum through the scarcely evident capsules is less 

 than that through the bases of the first molars. The base of pm*, 

 which is hidden by the incisor capsule in Geocapromys, often forms 

 an obvious external swelling in Capromys (as in pi. I, fig. 2). 



These characters indicate that the members of the two genera have 

 been developing along consistently different lines. In Capromys the 

 incisor root has pushed back to a position where more advance is 

 prevented by contact with the base of pm* ; in Geocapromys its posi- 

 tion is such that it could be extended much farther back in a capsule 

 lying along the outer surface of the molar shafts as in Spalacopus. 

 The Capromys condition is nearly paralleled in Octodontomys. In 

 correlation with the position of the incisor roots the molar roots are 



