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_: [M] _V._ 154; _C._ 58 = 212 _Coronella austriaca_: [F] _V._ 165; _C._ 48 = 213 _Vipera berus_: [M] _V._ 138; _C._ 35 = 173 _Vipera berus_: [F] _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.