Viper
2 of the 7 encyclopedias on this shelf carry an entry for Viper. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
a genus of venomous snakes, representative of the family Viperidæ. This family includes many important forms-e. g., the common adder (Vipera or Pelias berus); the asp (V. aspis), extending as far N. as Sweden; the African horned viper (V. cerastes) and puff adder (Clotho or Echidna arietans); the Indian daboia or Russell's viper (Daboia russellii); and the Indian Echis carinata. The head is relatively broad, somewhat triangular, and generally covered with scales; the eye has a vertical pupil, and there is no pit between it and the nostril; the maxilla bears on each side one functional fang, usually with several reserve fangs beside it; the poison is virulent. The vipers are widely distributed through Europe and Australia; the majority are African. As far as is known they are viviparous. The common viper or adder is the only poisonous snake indigenous to Great Britain. in. It lives especially on dry heaths and waste places, often among stones and brushwood; it is commoner in Scotland than in England, and does not occur in Ireland. It is widely distributed throughout Europe. Often confused with the innocent grass snake (Tropedonotus natrix), it may be distinguished by its markings. It has two diverging marks between and rather behind the eyes, a spot on each side of the hinder part of the head, a row of confluent rhomboidal spots running zigzag along the upper surface of the whole length of the body and tail, and a row of small irregular, almost black, triangular spots on each side. The under parts are of a lead color. The characteristic markings are almost invariable, but the ground color varies considerably, from nearly olive, rich deep brown, or brownish-yellow, to almost black. Thus in some parts of England a black viper is occasionally met with; its ground color a rich black, and the markings of a more intense black than the rest. There is also the "red" and the "blue-bellied," and an almost white viper, with black markings. The viper seldom exceeds two feet in length. It feeds on mice, frogs, small birds, and other small animals, which are killed by its poison fangs, and swallowed entire. It hibernates during several months of the year, and several may then be found twined together in a torpid state. It is a good swimmer, and may occasionally be seen on lakes, such as Loch Lomond, crossing from one island to another. The young are produced in early summer, from 10 to 15 or more at a birth. The eggs have soft, thin envelopes, and are hatched within the oviduct. The young viper is coiled up so closely in the egg as to appear almost a solid mass, but the moment it is set free it is active, and ready to throw itself at once into an attitude of defense. German
Aiton's Encyclopedia (1910)
a family of venomous serpents of the Old World, closely allied to the rattlesnake family of the New World. The name is Latin, meaning to bring forth alive. The young are produced alive, instead of being hatched from eggs. The common English name is adder, meaning simply a snake. As the adder is the only vicious serpent found in Great Britain , the translators of the King James Bible adopted the term. The common viper is about two feet long. It is of a yellowish brown, with a row of oblong black spots running lengthwise of its back, and a row of spots on each side. The head is oval with a blunt snout. The viper is the only venomous serpent in the greater part of Europe. It is neither common nor dangerous. Its bite is painful, but not fatal. Its reputation, however, accords well with the saying, "Look not upon the wine when it is red, when it giveth his colour in the cup, when it moveth itself aright. At the last it biteth like a serpent, and stingeth like an adder." Proverbs xxiii:31-32.