When using the terms mice and rats, we usually mean small ani-mals with a long scaly tail. These names are applied mainly to rodents, but not exclusively. In various languages, shrews are also called mice (e.g. German Spitzmäuse), as are bats and some marsu-pials (marsupial mice). Even one carnivore, the Egyptian mongoose (Herpestes ichneumon), is also called Waoh’s rat. First of all, it would be useful to figure out which of them are “mice” and which are “rats”. Actually, there is no fundamental dif-ference between mice and rats: a rat is a big mouse, and a mouse is a small rat. As we will see below, most murid rodents are interme-diate in size and are called rats or mice arbitrarily.In order avoid repeating the words “mice and rats” every time, we can introduce a general term covering all size variants: muridoid (Miljutin, 1992a, 2010). It is derived from the Latin name for the murid family, Muridae, and the suffix “-oid”, indicating similarity.A brief definition of the term is as follows: A muridoid is an animal that has a mouse-like body structure. This definition will, of course, remain incomprehensible and useless, if we do not specify what kind of body structure can be considered “mouse-like”. This should be the body structure that most murids have, for example, the well-known human companions – the house mouse and the brown rat. On the other hand, the features characteristic of this structure should also be present in the most phylogenetically dis-tant forms that we associate with mice and rats, such as the least specialized opossum species, like Monodelphis. Identifying external features common to true mice and opossums that are also charac-teristic of other mouse-like mammals, led me to develop the fol-lowing definition. Muridoids are four-legged, morphologically plantigrade ani-mals, without protective armour or flying membrane; their hind legs are longer than their front legs, and the digits of all limbs are mostly unfused and not shortened, and usually end with claws; their tails are oval in cross section, are equal to or exceed half the head and body length, and are covered either with bare skin (with or without scales) or short fur without a tuft at the tip.This definition refers to animals, but may be the muridoid struc-ture is found only in mammals? At the present time this is true, but in the Mesozoic it could have been and obviously was otherwise.It should be explained that by morphologically plantigrade condi-tion I mean extension of the soles of the feet to the wrist and heel, but not necessarily that the animals move on a full sole. Thus, the brown rat, for example, rests on full soles when stationary, but when moving rests on the soles of the forefeet and only on the digits of the hind feet. Such locomotion is also characteristic of other muridoids. In a functional sense, they are palm-digitigrade animals(Kuznetsov, 1999).