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.
As may be seen by a comparison of the first figure with the second, the arrangement of the head-shields is essentially different from that which prevails in the Colubrids and the majority of other snakes.
The second type is exemplified by the head of a member of the genus _Zamenis_.
In the descriptions, temporals 2 + 3 means two superposed temporals in the first row, three in the second. The internasals and the temporals, and the loreal and the preocular, are sometimes absent, and the prefrontal or the internasal may be single. One or two large shields are in rare cases present behind the parietals, and are called occipital.
[Illustration: FIG. 1--HEAD OF _Typhlops braminus_. (From "Fauna of British India")
_f_, Frontal; _ip_, interparietal; _l_, labial; _n_, nasal; _o_, ocular; _p_, parietal; _po_, preocular; _prf_, prefrontal; _r_, rostral; _so_, supraocular.]
A breaking up into smaller shields takes place in many snakes. In the Pythons, for instance, the frontal may be divided into two by a longitudinal cleft, and separated from the prefrontals by small shields. In some Vipers, such as _V. berus_ and _V. ursinii_, in which the frontal and parietals, though reduced in size, usually preserve their primitive condition, the former is normally separated from the supraocular by a series of small shields, and the internasals and prefrontals are broken up; in these snakes the small shield or shields behind the rostral are termed "apical," and those on the upper edge of the snout are termed "canthals." The shield which, in Vipers, separates the rostral from the nasal is called "naso-rostral." Allusion has been made above to the scaly dermal appendages which terminate the snout in certain genera. Some Viperidæ are furnished with horn-like erect spines above the eyes or at the end of the snout, which add greatly to their sinistral appearance.