The distinction of the European species is one of considerable difficulty, owing to their close relationship and the presence of intermediate forms connecting them. Matters being so, it seems curious that the Common Adder should have been regarded by so many authors as generically distinct from the Asp Viper, under the name of _Pelias berus_. It is highly probable that hybrids are produced in those districts where two species coexist, as in some parts of France, North Italy, and Austria. 21. VIPERA URSINII, Bonaparte Orsini's Viper _Form._--Short and stout. Snout obtusely pointed, flat above or with the canthus slightly raised. Eye very small, usually smaller than the nasal shield, its horizontal diameter usually not exceeding its distance from the posterior border of the nostril, its vertical diameter often less than and rarely exceeding its distance from the mouth. Length of tail seven to eight times in total length in males, nine and a half to twelve times in females. [Illustration: FIG. 33 (From Proceedings of the Zoological Society, 1893)] _Head-Shields._--Rostral as deep as broad or slightly deeper than broad, visible from above, in contact with one apical shield (rarely with two). Distinct frontal and (usually) parietal shields, the former once and a half to once and two-thirds (rarely once and one-third) as long as broad, as long as its distance from the rostral or the end of the snout, and nearly always longer than the parietals; the latter always in contact with the former, rarely broken up into small shields. Four to seven small shields on the snout between the canthals, of which there are two on each side. Supraocular well developed, extending posteriorly beyond the vertical of the eye, separated from the frontal by one to three shields, very rarely in contact with it. Six to ten scales round the eye, usually eight or nine, the upper preocular usually in contact with the nasal; a single series of scales between the eye and the labials. Nasal single. Temporal scales smooth. Upper labials six to nine, usually seven or eight, usually third or third and fourth below the eye. Three (rarely four) lower labials in contact with the chin-shields, of which there is but one pair. _Scales_ in nineteen (rarely twenty or twenty-one) rows, with two apical pits, strongly keeled on the back, less strongly on the sides, outer row smooth. Ventral shields 120 to 135 in males, 125 to 142 in females; anal entire; subcaudals 30 to 37 in males, 20 to 28 in females. By adding the subcaudals to the ventrals in a hundred specimens, the total numbers are 153 to 169 in males, 150 to 168 in females.