Another distinguishing feature is the absence of ventral scales in most of the species. In land snakes we saw how admirably adapted are the broad, ventral plates for assisting those reptiles over rough surfaces, as affording hold; but the _Hydrophidæ_ requiring no such aid in a fluid, those scutæ would be useless; they are therefore, excepting in one or two species, entirely absent, or but slightly developed, and the belly is ridged instead, like the keel of a boat. [Illustration: Portion of the under side of a sea snake, above and below the anus, with no distinction in tail scales.] The nostrils are small, placed horizontally on the top of the snout, as in the _Homalopsidæ_, and in most of the sea snakes they are contiguous. They are, moreover, furnished with a valve, which is under control of the will, opening to admit air, and closing to exclude water when diving. For, be it remembered, these marine reptiles breathe through their nostrils even more entirely than terrestrial snakes, the latter being better able to indulge their yawning propensities, or to occasionally respire slightly, and through parted lips and the tongue chink as well. Sea snakes, on the contrary, not requiring the continual use of their tongue to feel and explore surroundings, and not using it below water, are not provided with the little centre chink for its exsertion; but the middle plate of the upper lip, _i.e._ the ‘rostral shield’ (see illus. p. 238), is altogether of a different form. Indeed, the centre plates or shields in both lips are conspicuously modified, the upper one often inclining downwards in a point which fits into the lower one shaped to receive it, so that the mouth is firmly closed to keep out the water. Less required, the tongue is shorter and less developed, the tips are less hair-like, as only these, if at all, are exposed to the sea water, and a very small notch on each side of the pointed rostral shield of some permits the slight egress of these tips. When out of their natural element, the tongue is brought into more active service, for then the bewildered reptiles require its assistance, and it is then seen to be exserted as in land snakes. Their lungs extend the whole length of the body to the anus, and by retaining a large supply of air, these animals are enabled to float easily, as they do for a long while on the surface of the calm tropical seas, not only while sleeping, as mentioned in the chapter on hibernation, but in pure enjoyment, and probably in the lazy _postprandial_ condition. As has been already stated, the eyes of sea snakes are adapted to see better through the medium of water than through the brilliant atmosphere of their native latitudes. They are very small, and soon blinded by light; consequently, though among the swiftest and most gracile of serpents in their native element, the movements of the _Hydrophidæ_ on land are uncertain and ‘maladroit.’