‘_Fresh-water snakes_’ form the fourth, and ‘_Sea snakes_’ the fifth of the five groups into which Dr. Günther has separated the ophidian families; but the gradations between the land and the fresh-water species, and between the latter and the salt-water snakes or the true _Hydrophidæ_, are, like all other herpetological features, extremely close. There are water-loving land snakes and land-frequenting water snakes, that is, those which are equally at home in both. In the true water species, however, we find modifications of ordinary rules which show them to be peculiarly protected and adapted for an aquatic existence. One notable characteristic in all, both salt-water and fresh, is the position of the nostrils on the top of the snout, and in many these are protected by a valve which closes at will. As air-breathing animals they must come to the surface, but the timid, stealthy ophidian instinct which seeks to hide from observation can be indulged even in the water, with the nostrils so situated that only a very small surface of the head need be exposed. Could we examine the interior of the mouth we should doubtless find some slight variation in the position of the glottis also. In a foregoing chapter we saw that the trachea opens exactly opposite to and close behind what Dumeril calls the ‘arrière-narines;’ ‘leur glotte qui est à deux lèvres et qui represente un larynx très simple, s’ouvre dans la bouche derrière le fourreau de la langue ... elle s’élève pour se presenter dilatée sous les arrière-narines.’[71] The glottis of water snakes must have a still more upward direction to present itself to those air passages. Perhaps water snakes do not require to yawn so frequently as is the habit of their terrestrial relatives; and if they do, it must be a rare privilege to be able to inspect the process, as one can so frequently do with the pythons and vipers at home. Our authorities do not give us much information on this point.[72] Their moderately long tapering tail is used as a propelling power. Exteriorly, too, water snakes have smooth non-imbricated scales, though exceptions exist in those species which frequent both land and water, as the _Tropidonoti_, a large family of which our common English ring snake is a member, and which, as their name denotes, have all keeled scales, from τρόπις, τρόπιδος, _a keel_. These, also, can elevate their ribs, and so flatten the body in the water, another assistant in swimming.