Black specimens occur, more or less frequently, all over the habitat of this species, and are often referred to as _V. prester_, Linnæus. A distinction has to be made between individuals which are black through darkening of the ground colour, and such as are thus coloured through expansion and confluence of the markings. The latter are males, and among them we may find intermediate stages showing how this melanism is brought about; in one case the black of the back is separated from the black of the sides by a narrow light brown wavy stripe, the remains of the ground colour. When, as in all females, and occasionally in males, the black is the result of a gradual darkening of the ground colour, the typical markings may still be detected under certain lights. Some specimens (from Schneeberg, Lower Austria) are black, with scattered golden dots, or of a dark mahogany brown speckled with yellowish. In nearly all the black specimens at least a few dots of whitish are visible on the lips, and of yellow under the end of the tail. Most of the variations enumerated above occur irrespective of the geographical distribution. Two forms, however, deserve to be regarded as ill-defined local races: the var. _seoanei_, Lataste, from North-Western Spain, in which, in addition to the peculiarities of coloration mentioned in the description, the canthus rostralis is frequently more distinctly raised, and the frontal and parietal shields are often disintegrated into scales; and the var. _bosniensis_, Boettger, from Bosnia, Carniola, and Carinthia, which is sometimes very suggestive of, and has been taken for, the typical form of _V. aspis_, having like it, though not at all constantly, two series of scales between the eye and the labials, and the zigzag band replaced by a series of dark bars across the back. The var. _pseudaspis_, Schreiber, from the plains of Sclavonia, described as straw yellow above, with narrow dark cross-bars, is hardly separable from the var. _bosniensis_. _Size._--_Vipera berus_ is said to reach very exceptionally a length of 2 feet 11 inches. The largest specimen in the British Museum (from Belgium) measures 2 feet 3-1/2 inches: the largest British specimen 2 feet 3 inches. Both these specimens are females. The largest male measures 2 feet 2 inches.