They have long hind limbs, large eyes and ears; and in correlation with the latter an enlarged auditory bulla to the skull, which is hollow and divided into a tympanic and a mastoid portion.
Tympanic bullae are always present and generally large; in some genera, as in the gerbils (Gerbillinae) and jerboas (Jaculidae), there are supplemental mastoid bullae which form great hemispherical bony swellings at the back of the skull (fig.
In the highly specialized mastoid region of the skull, the North American Oligocene Protoptychus approaches to Dipopodomys, while the contemporary Gymnoptychus and Entoptychus likewise appear referable to the Geomyidae.
The first of these, or Geomyinae, is characterized as follows: Incisors broad; mastoid not appearing on the top of the skull; eyes small; ears rudimentary; limbs short, subequal.
X P11,1x, premaxilla; Mx, maxilla; Ma, malar; Fr, frontal; L, lachrymal; Pa, parietal; Na, nasal; Sq, squamosal; Ty, tympanic; ExO, exoccipital; AS, alisphenoid; OS, orbito-sphenoid; Per, mastoid bulla.
The following are the characters of the second sub-family, Heteromyinae: Incisors narrow; mastoid appearing largely on the top of the skull; eyes and ears moderate or large; hind-limbs and tail elongated.