‘The active instruments are a pair of fangs.’ ... They are ‘somewhat conical and scythe shaped, with an extremely fine point; the convexity looks forward, the front downward and backward’ (referring to the slight double curve in the Crotalus fang as shown in the illustration, p. 360). They are hollow by folding, ‘till they meet, converting an exterior surface first into a groove, finally into a tube.’ ... The fang is ‘moveable, and was formerly supposed to be hinged in its socket. But it is firmly socketed, and the maxillary itself moves, which rocks to and fro by a singular contrivance. The maxillary is a small, stout, triangular bone, moveably articulated above with a smaller bone, the lachrymal, which is itself hinged upon the frontal.... This forward impulse of the palatal and pterygoid is communicated to the maxillary, against which they abut, causing the latter to rotate upon the lachrymal. In this rocking forward of the maxillary, the socket of the fang, and with it the tooth itself, rotates in such a manner that the apex of the tooth describes the arc of a circle, and finally points downward instead of backward. This protrusion of the fang is not an automatic motion, consequent upon the mere opening of the mouth, as formerly supposed, but a volitional act, as the reverse motion, viz. the folding back of the fang, also is; so that in simply feeding the fangs are not erected.’ (But I think I may affirm positively that sometimes the vipers do use their fangs in feeding. When they open their mouths—or rather the jaws alternately very wide—I have seen first one and then the other fang occasionally engaged in the food and again disengaged unsheathed. On other occasions the fangs have been folded. In some large African vipers, the ‘River Jack’ and others that were in the Society’s Gardens a few years ago, I was able to observe this easily.)