Word Origin and History for semi-Expand before vowels sem-, word-forming element meaning half, part, partly; partial, imperfect; twice, from Latin semi-half, from PIE *semi-half (cf. Sanskrit sami half, Greek hemi-half, Old English sam-, Gothic sami-half).
Well-known rodents include mice, rats, squirrels, prairie dogs, porcupines, beavers, guinea pigs, hamsters, gerbils and capybaras. Other animals such as rabbits, hares, and pikas, whose incisors also grow continually, were once included with them, but are now considered to be in a separate order, the Lagomorpha.
Muridae (family Muridae), largest extant rodent family, indeed the largest of all mammalian families, encompassing more than 1,383 species of the âtrueâ mice and rats. Two-thirds of all rodent species and genera belong to family Muridae. The members of this family are often collectively called murids, or muroid rodents. 1  The 300 genera of muroid rodents are classified within 18 subfamilies, but more than 200 of them (and nearly 1,000 species) belong to only two subfamiliesâSigmodontinae (New World rats and mice) and Murinae (Old World rats and mice).