Changes in the sequence also have been necessitated by the numerous systematic modifications called for by the various authorities cited in the generic accounts. The various groups of murids differ substantially in form and function. Total length ranges from less than 100 mm in Baiomys to 800 mm or more in Phloeomys and Cricetomys. Murid molars may be rooted or rootless and either laminate, cuspidate, or prismatic. Murids are found in a wide variety of habitats.
Most species are primarily terrestrial, but some are arboreal, fossorial, or semiaquatic.
They shelter in tunnels or crevices, under logs or other suitable objects, in hollow trees or logs, or in nests built on the surface or in bushes or trees. They may be diurnal or nocturnal and are usually active throughout the year. Most genera feed on plant material and invertebrates. Some take small vertebrates, including fish. Seeds and other plant matter are sometimes stored for winter use.