Another caprice is seen in the carinated or keel-shaped body scales, which are found in venomous and non-venomous, land and water, ground and tree snakes indifferently; though I think one may be safe in affirming that none of the true vipers have unkeeled and polished scales. Nicholson has observed that in several allied species, some have and some have not the keel, and that those without do as well as those with. ‘The history of the keel is not known,’ says this author. In appearance it reminds one of the mid-rib of a leaf or of a feather, and may probably be an inherited feature in common with birds whose reptilian ancestry in process of ages had fluttered their scales into feathers. In fact, in many snakes where no keel is found, there is some slight indication of a centre line, even if it take the form of a groove or depression. In the _Tropidonoti_ the keel is so developed as to distinguish the group; yet many with keels have comparatively smooth skins. The carinated scales of vipers (from _carina_, a keel) are sharply defined, like the keel at the prow of a ship, or like the breast-bone of the swift-flying birds which Mr. Sclater, in one of his zoological lectures, described as the carinate birds. It is these sharply-defined, stiff; and dull scales belonging to the vipers which produce the rustling noise when the snake is agitated, as described in the little Indian _Echis carinata_ in the chapter on hissing. In the _Cerastes_ I have witnessed the same agitated convolutions accompanied by the audible rustling produced by the rough scales. See illus. p. 317. What are called ‘horns’ in some of the African vipers are curiously-modified scales, which, under close examination, present the appearance of half-curled leaves, sometimes of ears, like those of a rabbit or a mouse. Being only cuticle, and liable to injury, these ‘horns’ vary in size and colour as well as form. [Illustration: The sloughed horns of _Vipera nasicornis_ (exact size).] The accompanying figure is from the slough of the _Vipera nasicornis_ of the coloured illustration. They were not reversed in desquamation, but came off with a portion of the fine spiny head scales. They were so dry and shrivelled at the time, that it is hard to conceive how they could possibly be reversed, the rest of the bristly head-scales peeling off in pieces. Yet we cannot conclude from this that the horns are _never reversed_ in sloughing; the individual in question having undergone long captivity in a close box during her journey from West Africa, and arriving at the Zoological Gardens in such a miserable plight that it was difficult to distinguish species or colouring for many days. In this condition she remained for five weeks, when one fine Sunday afternoon she presented the Society with forty-six viperlings. [Illustration: VIPERA NASICORNIS AND YOUNG ONE. AFRICA. Mother over five feet long, Viperling 9 inches.]