* FAMILY. 
SACCOMYIDAE. 
With large and distinct external cheek pouches. Upper lip hairy, not cleft. Molars, rooted or rootless, = Temporal 
bone enormously developed. No post-orbital process of the frontal. The ante-orbital foramen elther wanting, as such, or 
appearing far anterior to the base of the zygoma. The upper corner of the postero-inferior angle of the lower jaw nearly 
horizontal. A tubercle outside of the condyloid process, indicating the posterior extremity of the incisor. Tibia and 
fibula united. Generally but five toes, with distinct claws on each foot. Fore claws longer than hinder. Coecum distinct. 
Pelage composed of stiff hairs, without any under fur. 
The above characters serve to define one of the most natural families of the Modentia, 
although the component genera have been widely separated by different authors. In the 
external cheek pouches there is no other family which exhibits any approach to it. These open 
outside of the mouth and are of variable depth, and lined with short hairs to the bottom. 
When inverted and dried, they look like sacks on each side.of the head. While other charac- 
teristics are shared by one or other of the different families of Rodents, the Saccomyidae possess 
the common character of the external cheek pouches, as distinguished from all others. From 
the Leporidae, the lack of a second pair of upper incisors, behind the first, will at once distin- 
guish them, independently of the other totally irreconcilable features. The Hystricidae, 
although having the same dental formula, may be at once known by the very large ante-orbital 
foramen in the zygomatic process of the maxillary as well as by the separated tibia and fibula. (?) 
The having four lower molars instead of two or three, with the lack of the ante-orbital foramen 
separates them from the Muridae, although they agree in the united tibia and fibula; this, 
however, separates them again from the Sciwridae. The nearest approach is seen, perhaps, in 
the beavers, Castorinae, which, differing altogether in external form, have certainly a close 
resemblance in the skull. The most striking points of distinction are seen in the deep malar 
bone which extends up along the zygomatic plate of the maxillary to the lachrymal and back- 
ward posterior to the molar. Many others, however, could be enumerated. 
There are two very distinct sub-families in this group, typified by Geomys and Dipodomys, 
agreeing in the family characters, yet very distinct from each other and especially in external 
form; the one being the most clumsy, thickset, and sluggish of American rodents; the other 
the most delicate, graceful, and agile. Yet the differences are owing to the greater or less de- 
velopment merely of common conditions. These may be briefly characterized as follows: 
Geomyinae.—Hind legs very short; fore claws enormously developed ; tail short; mastoid 
portion of temporal not forming part of upper surface of skull. 
Saccomyinae.—Hind legs and tail very long; fore feet moderate; mastoid bone very large, 
occupying much of upper surface of skull. 
Brandt, in his recent paper on the classification of the Rodentia in Beitrige zur nihern 
Kenntniss der Siugethiere Russlands, 1855, 188, establishes a family of Sciwro-spalacoides to 
contain Geomys and Thomomys, as constituting a connecting link between the Sciwridae and a 
family of Spalacoides, typified by Spalax, Siphneus, Ellobius, &c. THe dissents from the views 
