666 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxiii. 



In ^ummino- up, the old tA\\\\\j Dipodidce is seen to l)c composed of 

 two clearly defined though somewhat related families, of which Z<ij»is 

 is typical of the one and iJt/nis of the other. The antorbital foi-amen 

 and its subdivision for the nerve and lachrymo-malar articulation are 

 the only striking points of similarity between the two families, Init 

 otherwise the skulls are widely different and each homogeneous in its 

 own famil3\ 



The Z( I pod like are at once recognized ])y the five separate metatar- 

 sals, free cervical vertebra^ and general nnirine aspect of the skull. 

 It is composed of two easily separable subfamilies. 



Zapudhua., wnth the enamel of the molar teeth thrown into folds a-nd 

 the crowns presenting a generally smooth surface; upper incisors 

 grooved; skull less murine; zygoma heavier and less oblique palate 

 concave posteriorly. It contains the three genera, Zajms, Ncqyaeoza- 

 pus^ and Eozapyus. 



Smintlumv, with the enamel of the molar teeth in an entirely differ- 

 ent pattern, and above folded in opposite loops so that there seems to 

 be four cusp-like processes on each tooth; the upper incisors without 

 grooves; and slenderer skull and zygoma. It contains the genus 

 S/jtinthus and possibly the fossil genus J^ouii/s, which is usually referred 

 to this group. 



The Dqiodidcti are to be recognized by the fusion of the three middle 

 metatarsals into a '"cannon"' bone, longer hind limbs, a tendency 

 toward consolidation of the cervical vertebrae, as well as a totally dif- 

 ferent form of skull, much laterally expanded. It seems to be readily 

 separable into the three following groups, of which the first two 

 should take subfamily rank, the D'ljyodlma in contrast to the third 

 group containing EuchoreutcK. 



D'qjm group with Dlpux and its subgenera, hind foot with three 

 digits; cervical vertebra anchylosed; mastoid considerably inflated; 

 upper incisors grooved; no small premolar; antorbital canal for nerve 

 complete. 



Alactaga group, with Ahtetaga and its subgenera, hind foot with 

 more than three digits, but lateral ones much shortened; cervical ver- 

 tebra' not completely fused; mastoids not ipuch inflated; upper inci- 

 sors without grooves; and a small premolar present above; antorbital 

 canal for nerve not fully complete. Platycercomys Avithout the small 

 premolar probably l)elongs to this group. 



ExchoreuthuB is at once told from the preceding by the posterior 

 slope of the zygoma and more elongated skull and intcrorbital con- 

 striction; no root-cap for incisor on side of mandible; posterior pala- 

 tine foramina very large; hind foot with five digits; upper incisors 

 not grooved; upper premolar present. 



