A number of characters link most muroids. Not surprisingly, even the most basic characters are subject to continuing evolutionary change; most of the characters listed as diagnostic in the next paragraph do in fact show some variation within the group. All, however, are believed to have characterized primitive muroids.

In the   skull of muroids, the   infraorbital foramen, which primitively transmits nerves to the rostral region of the skull, lies mostly above the   zygomatic plate. It is enlarged above for the passage of a slip of muscle that inserts on the lower jaw, and narrowed in its lower region, through which pass nerves and blood vessels en route to the rostrum. The foramen thus has a distinctive "keyhole" shape in most forms (but the narrow ventral portion is lost in a few species). The zygomatic plate, formed by the anterior base of the   zygomatic arch, is broad and a conspicuous feature of the cranium. From it arise other parts of the same muscle (the   masseter) that passes through the infraorbital foramen. The   jugal, one of the bones that participates in the zygomatic arch, is small and does not contact the   lacrimal. The   frontals are constricted above the orbits and there is no   postorbital process or bar. Posteriorly, an   interparietal bone is present and usually conspicuous.

The lower jaw is   sciurognathus. As in all rodents, one upper and one lower   incisor are always found on each side of the jaw, and   canines are always absent. Following the incisor is a   diastema.   Canines and   premolars are never present. No more than three   molars occur on each side, but this number is sometimes reduced to two or even one. The nature of the molars (shape, size, surface structure, number of roots) varies greatly.

Four clawed digits are found on each forefoot (the pollex or "thumb" is small and bears a nail); the hind foot in most has five clawed digits (but sometimes the hallux or first toe has a nail). Other external features (ears, eyes, tail, pelage, etc.) are extremely variable. To compound this variability, some populations of some species are polymorphic, and some exhibit sexual dimorphism in body size.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry ; polymorphic

Sexual Dimorphism: sexes alike; female larger; male larger