Most snakes have a longitudinal groove on the chin (mental groove) to allow for the distension caused by the lateral movements of the rami of the lower jaw.

In the Typhlopidæ, the head passes gradually into the vermiform body, and the small mouth is situated on the under surface of the projecting snout; the head so resembles the extremely short tail, and the mouth is so similar in shape and position to the vent, which is close to the posterior extremity of the snake, that such creatures are often believed by non-critical observers to have a head at each end. The eyes are very small, and covered over by the semi-transparent head-shields, or they may be completely concealed. There is no mental groove. It is much the same with the Glauconiidæ, which have, however, a somewhat less abbreviated tail. In both, the nostrils often open on the lower side of the snout, which may be excavated so as to appear hooked in profile, or may be provided with a sharp cutting horizontal edge.

Snakes are covered with epidermal folds in the form of scales and shields, the shape and arrangement of which affords important characters for their classification. Dermal ossifications are absent.

The scales on the body are usually elliptic or lanceolate and imbricate, forming straight longitudinal and oblique transverse series, and they are replaced on the belly and under the tail by transverse shields mostly corresponding in number with the series of scales, and also with the vertebræ. The body of the Typhlopidæ and Glauconiidæ is uniformly covered with polished, closely adherent, rounded, overlapping, sub-equal scales, without even an indication of ventral shields. In some of the Acrochordinæ, aberrant aquatic Colubrids, the scaling consists, above and beneath, of small juxtaposed, sometimes spinose granules, the skin being suggestive of the shagreen of sharks. In the marine snakes of the subfamily Hydrophiinæ, the ventral shields are often absent or merely indicated, and the scales are mostly juxtaposed or feebly imbricate, sometimes tetragonal or hexagonal, and occasionally studded with spinose tubercles. In the more typical Ophidia the imbricate scales may be long and narrow or short and broad, with every intermediate step between the two extremes; smooth or furnished with a longitudinal ridge or keel, or even several keels; nearly equal in size or with the median or outer series more or less enlarged, the longitudinal series in odd, rarely in even number; instead of running in longitudinal series parallel with the axis of the body, as is the rule, they are sometimes disposed obliquely, and among those in which we meet with this peculiarity several genera are further remarkable in having some of the oblique lateral scales furnished with a serrated keel, to which we shall again allude in the chapter on Habits, when dealing with the rustling sounds produced by certain snakes. The number of longitudinal series of scales on the body varies from 10 (Herpetodryas) to nearly 100 (Python, Boa); in the European species from 17 (Contia modesta) to 50 (Eryx jaculus). The scales are sometimes furnished near the end with one or two shallow impressions, termed “apical pits,” which afford indications for the distinction of genera and species; unless of a lighter or darker colour, as is often the case, these pits are not always easy to see, except in a strong light and with the aid of a powerful magnifying glass.

The ventral shields, also called “gastrosteges,” usually occupy the whole width of the belly; but they may be much narrower—in Eryx, for instance. They are sometimes bent at an angle on the sides, and this angle may even form a sharp keel, accompanied by a notch in the posterior border, corresponding to the keel, as in several of the more arboreal genera of Colubrids. The shields under the tail, termed subcaudals or “urosteges,” are sometimes similar to the ventrals, but more often disposed in pairs; in certain species or individuals some of the subcaudals are single, and the others paired. When the number of subcaudals is given in the descriptions, each pair is reckoned as one, and the conical or spine-like shield which caps the end of the tail is not included. These numbers afford important characters for the definition of species, and sometimes also for the distinction of sexes. The subcaudals are nearly always much fewer than the ventrals, but the difference is often not so great in the males as in the females, the tail of which is usually shorter in proportion to the body. It is noteworthy that in many species, if the number of subcaudals (C.) be added to that of the ventrals (V.), the total is nearly the same in the male as in the female, however much the respective numbers may differ when taken separately. The following figures may be given by way of example, taken from British specimens:

Coronella austriaca:♂V. 154;C. 58 = 212
Coronella austriaca:♀V. 165;C. 48 = 213
Vipera berus:♂V. 138;C. 35 = 173
Vipera berus:♀V. 144;C. 29 = 173

Although this rule is by no means universal, and does not apply at all to some species, it will be found to hold good in many cases, and is of interest in showing that the changes that have taken place in the vertebral column (the vertebræ corresponding in number to the shields), according to the sexes, have been by a modification of the character of the segments about the anal region, a conversion of trunk vertebræ into caudals, or vice versa. In dealing with certain species—of Vipers, for instance—it is important, for systematic purposes, to keep the counts of shields distinct for the two sexes.

The shield which covers the vent, the anal shield, is either single or divided into two.

Some snakes have the head covered with scales or small tubercles similar to those on the body, but in the great majority the lepidosis is in the form of large symmetrical juxtaposed shields, the shape, proportions, and number of which furnish some of the most important characters for the distinction of genera and species. These head-shields belong to two primarily different types, from each of which all further modifications may be regarded as derived by alteration in shape or by disintegration. The first type is that shown by the Typhlopidæ and Glauconiidæ, which is explained by the figure on the next page.

The rostral, which is usually the largest of the head-shields, extends to the upper surface of the head, of which it may occupy the greater part. In the Glauconiidæ, the ocular usually borders the mouth.