NATURAL SCIENCES OF PHILADELPHIA. 277 



Saccomyid^e and Geomyid^b. Mastoid bone inordinately de- 

 veloped, occnp3'ing much of the occipital or superior surface of 

 the skull, or both. Occipital correspondingly reduced. No post- 

 orbital processes ; no anteorbital foramen as such, in its stead a 

 perforation in the side of the maxillary far forward and low down. 

 Z} T gomatic process of maxillary an expanded perforate plate. 

 Molars f . Root of under incisor more or less protuberant pos- 

 teriorly. Descending process of mandible an obliquely twisted 

 plate, projecting outward and upward. Large external fur-lined 

 cheek pouches with special muscle ; upper lip densely hairy, not 

 visibly cleft ; feet pentadactyle ; fore claws longer than hinder 

 ones. Pelage without under fur. 



It will be observed that the differences outweigh the resem- 

 blances, notwithstanding the higher taxonomic value of some of 

 the latter. The peculiar state of the temporal bone, the position 

 of the anteorbital foramen, the shape of the jaw, and the presence 

 of the pouches are the main common characters. In contrasting 

 the two families, the very peculiar genus Dipodomys offers the 

 strongest points of difference ; but when we come to consider 

 Perognathus and Cricetodipus, many of the expressions applicable 

 to Dipiodomys require modification. Perognathus is a link between 

 the two families, just as, in its own family, Cricetodipus is between 

 Perognathus on the one hand, and the extraordinarily modified 

 Dip>odomys on the other. 



This brings us to consideration of what if any division may be 

 made of the family Saccomyidee into groups of more than generic 

 value. The genus Dipodomys is so exaggerated in some of its 

 peculiarities that, as it seems to me, its features may properly be 

 set over against the characters which Perognathus and Cricetodipus 

 share together. The leading antitheses may be indicated in the 

 following manner : 



Subfamily Perognathidin^e. Subfamily Dipodomyin^e. 



{Perognathus and Cricetodipus.) {Dipodomys alone.) 



Molars rooted. Molars rootless. 



Anterior molar with a lobe in addi- Anterior molar a simple prism, 

 tion to the main prism. 



Skull half as wide as long, and Skull two-thirds as wide as long, 



two-thirds as high as wide. and half as high as wide. 



Mastoids moderately developed Mastoids extraordinarily developed 



[ for this family). (even for this family). 



