This group gathers taxon/Mammal Families subcategory members that are themselves mammal families belonging to the order Rodentia, the largest mammal order by number of species, or the closely related order Lagomorpha. It spans familiar families such as Sciuridae, the squirrel family, Muridae, the largest rodent family, encompassing most mice and rats, and Castoridae, the beaver family, alongside more specialized families such as Caviidae, the cavy family that includes guinea pigs and the capybara, the largest living rodent, Hystricidae and Erethizontidae, the Old World and New World porcupine families respectively, and Leporidae, the rabbit and hare family that gives Lagomorpha its common name despite sitting in its own separate order from the true rodents. Every rodent family here shares a pair of continuously growing incisors at the front of the jaw that must be worn down by gnawing, the defining trait that gives the order its name.