‘Serpents are naked,’ says Günther—that is, they have no separate epidermal productions in the way of fur, feathers, hair, or wool, and all the variations of form in scales are but the folds of the epidermis.[90] The ‘variations of form’ include, therefore, the appendages above mentioned. The heads of most snakes are covered with non-imbricated plates or shields. The form and position of these shields are in a great measure used in classification; ‘are of the greatest value for distinction of species and genera.’[91] For this reason each and all of the head shields are specially named. Ophiologists differ slightly in distinguishing them as regards assigning the exact position of some of the shields, which, like all other ophidian features, vary in closely allied species. As, for example, while one naturalist may decide that a certain shield is exactly over the eye, another may consider it somewhat to the right or the left. Günther’s classification being the one now generally adopted, I copy the names assigned by him, and the diagrams given in his work. [Illustration: _Fig. 1._ Top of the head of a Colubrine snake. _r_, rostral; _f’_, anterior frontal; _f_, posterior frontal; _v_, vertical; _s_, supraciliary; _o_, occipital; _t_, temporal. _Fig. 2._ Profile of the same. _t_, temporal; _p_, posterior ocular or orbital; _a_, anterior ocular or præorbital; _l_, loreal; _n_, nasals; _uu_, upper labials; **, lower labials. _Fig. 3._ Under side of the same. **, lower labials; _cc_, chin-shields; _m_, mental or median lower labial.] It will be observed that some of these shields can be seen both in the profile and the others as well; as, for instance, the temporal and the labial or lip shields. The study of them is simplified by the initial letter of each name being used in reference to them. The names used also speak for themselves; as _mental_, the chin shield; _nasals_, near the nostril; _rostral_, the beak shields. Ophiologists in deciding species, etc., enumerate those which are more than a pair as ‘upper labials’ so many, ‘lower labials’ so many. In some snakes these shields are so large as to cover nearly the entire head; in others, they are almost inconspicuously small, or absent altogether, and much varied, as we shall see. In the vipers the head is generally covered with small, rigid, imbricated, or overlapping scales instead of plates, and in some the scales are so extremely fine and closely arranged as almost to represent short bristles. This is noticeable in the African ‘nose-horned viper’ (_Vipera nasicornis_), p. 322, where they present a curiously complicated structure. [Illustration: Magnified carinated scale. Magnified head-scale of _Vipera nasicornis_, of the coloured illustration.]