_Head-Shields._--Rostral as deep as broad or slightly broader than deep, rarely once and one-third as deep as broad, not or but scarcely visible from above. In addition to the supraoculars, three large shields, the frontal and the parietals, are as a rule present on the top of the head. Frontal as long as broad or a little longer than broad, rarely much longer than broad, once and a half to twice and a half as broad as the supraocular, from which it is as a rule separated by one to four shields, as long as or a little shorter than its distance from the rostral, as long as or a little shorter than the parietals. Parietals usually in contact with the frontal and separated from the supraoculars by small shields, but sometimes in contact with both, or separated from the frontal. Exceptionally, in specimens from Great Britain, Germany, and Austria, the parietals, or the frontal and the parietals, are broken up into scales, and this is more frequently the case in specimens from North-Western Spain (var. _seoanei_). Upper surface of snout bordered by six (rarely by five or four) small shields, viz., two apicals (rarely one), and on each side two canthals, the second of which is usually in contact with the supraocular; canthals very rarely united into one shield; the space between these shields covered by four to twenty flat or convex, juxtaposed scales, which very exceptionally are fused into a single large shield. Supraocular usually extending posteriorly beyond the vertical of the eye. Six to thirteen scales round the eye, usually eight to ten; two or three superposed scales, rarely two vertical series of scales, separate the preoculars from the nasal, which is single. As a rule a single series of scales intervenes between the eye and the labials; specimens with two series are of very exceptional occurrence (single specimens from Isle of Arran, Normandy, Southern Norway, and Carniola, in the British Museum), but there are occasionally two series except just below the centre of the eye. Upper labials six to ten, usually eight or nine; fourth or fourth and fifth (rarely third and fourth) below the eye. Temporal scales smooth, rarely feebly keeled. Three or four (rarely five) lower labials in contact with the single pair of chin-shields. _Scales_ in twenty-one (rarely nineteen or twenty-three) rows, with two apical pits, strongly keeled, those of the outer row smooth or feebly keeled. Ventral shields 132 to 150 (usually 137 to 147) in males, 132 to 158 (usually 140 to 150) in females; anal entire; subcaudals 32 to 46 (usually 35 to 40) in males, 24 to 38 (usually 28 to 33) in females. _Coloration._--It is characteristic of this species, contrary to the rule in snakes, to present such marked differences of colour, according to the sexes, that these can be distinguished in most cases from that character alone.