The tympanic process of the alisphenoid bone of the skull is short, not covering the cavity of the tympanum, nor reaching the paroccipital process.
The third sub-family is that of the Microtinae, or voles, which are distributed all over Europe, Northern Asia and North America, and are characterized by the tympanic bulla of the skull being filled with honey-combed bony tissue, the small size of the infra-orbital foramen, and the deep pterygoid fossa on the palatal aspect.
The infra-orbital foramen is generally narrow, and the tympanic bulla hollow.
The three remaining families of the Hystricoidea, of which one is African while the other two are chiefly South American, are very closely allied and often brigaded in a single family group. In the Capromyidae, which includes only the South American and West Indian hutias, the South American coypu and the African cane-rats, the tympanic bulla of the skull is hollow, the par-occipital process straight, the lachrymal small, and the cheekteeth rooted, with deep enamel-folds; the first front toe Leing occasionally absent.
Birds possess an ear-muscle which at least acts as a tensor tympani; it arises near the occipital condyle, passes through a hole into the tympanic cavity, and its tendon is, in various ways, attached to the inside of the membrane and the neighbouring extracolumellar processes.
The circular space on each side of the basi-temporal (bt.) is the opening of the anterior tympanic recess.
The hollow tympanic bullae, they have the clavicles imperfect, the first front toe opposable to the rest, the temporal region of the skull roofed with bone, and the crowns of the molars with cusps arranged in rows but eventually covered by a layer of enamel.
The tympanic forms a tubular meatus auditorius externus directed outwards and slightly backwards.
In the skull the lachrymal bone is large, the paroccipital process is directed vertically downwards and the tympanic bulla is hollow.
The tibia and fibula are united inferiorly, the tympanic bulla is hollow and the infra-orbital foramen narrow.