The Snakes of Europe

by George Albert Boulenger

Thoroughly aquatic snakes are often short and heavy, but some of the marine forms, or Hydrophids, may be extremely slender, with the posterior part of the body compressed. In some of these Sea-snakes the gracility of the anterior part, or “neck,” as it has been called, contrasts very strikingly with the great girth of the body towards the tail, and suggests a limbless Plesiosaur.

The tail, the part of the body behind the transversely cleft vent, is most frequently about one-fourth or one-fifth of the total length; but it may be much shorter, even reduced to a mere stump, as in the Typhlops, or, at the opposite extreme, enter for one half in the length of the snake, as in the African Xenurophis. This organ may taper gradually to a fine point; or end abruptly, as if mutilated; or terminate in a horny spine, such as we see in some of the Typhlops or in the Australian Death-adder, Acanthophis, or in a series of horny segments which are vibrated like a rattle, as in the well-known Crotalus of America, to which we shall refer again at the end of this chapter. In some of the burrowing Uropeltidæ, the very short tail is obliquely truncated, with indurated shields above, and acts as a trowel. And, finally, the marine snakes of the subfamily Hydrophiinæ are distinguished by a strongly compressed, oar-shaped tail, with rounded vertical outline. In a few forms, arboreal or aquatic, the tail is more or less prehensile.

Males generally have a longer tail than females, and the genital organs, which are lodged in its base, cause a swelling of that region which contrasts with the more gradually tapering extremity of the female, thus affording a means of distinguishing the sexes externally in the majority of snakes.

The rudimentary hind limbs of Boid snakes, to be mentioned further on in the description of the skeleton, terminate in a claw-like horny spur, which appears on each side of the vent in the male, and sometimes also, though less distinctly, in the female. These spurs are probably of use in facilitating the pairing, an explanation which appears the more plausible from the fact that the snakes provided with them have the copulatory intromittent organs destitute of the erectile spines which are present in most others.

The head varies in shape as much as the body. Although never actually compressed, except in the rostral region, it may be very narrow and elongate, whilst in the opposite extreme it may be strongly depressed, and so broad behind as to be abruptly defined from the anterior part of the body, or “neck.” This feature is very marked in some of the Viperidæ, and this has given rise to the incorrect generalization that poisonous snakes are distinguished from the harmless by a broad and flat head, notwithstanding the fact that some of the most dangerous, such as the Mambas, Cobras, and Kraits, have a comparatively narrow or small head, not or but slightly defined behind, whilst, on the other hand, the very opposite condition obtains in not a few of the harmless Colubrids.

Leaving the Typhlopidæ and Glauconiidæ aside for the present, snakes have a wide gape, cleft far beyond the vertical of the eyes, with, when closed, one or two notches in front for the passage of the protrusible, bifid tongue. In most snakes this chink is in the lower border of the rostral shield, capping the tip of the snout, and allows free passage to the whole tongue; in the Hydrophids, or Sea-snakes, there are two notches in the lower border of the rostral shield, through which only the bifid end of the tongue can be protruded. The eyes, varying from minute to enormous, are usually free from the surrounding shields, and may move under a transparent cap like a watch-glass, which appears to represent the lower eyelid of Lizards. The view as to this homology is derived from our knowledge of various conditions in certain series of Lizards of the families Lacertidæ and Scincidæ, where we find a transparent disc appearing like a small window in the movable lower eyelid, gradually increasing in size so as to occupy the whole of the lower eyelid, which finally becomes fused with the rudimentary upper lid and loses its mobility. In Ilysia and in most of the Uropeltidæ, the transparent disc over the eye is confluent with a thick horny shield of which it occupies the middle.

The pupil is usually circular or vertical, rarely horizontal. In some forms it is difficult to decide whether it is round or vertically elliptic; in others, like the Boas and Vipers, for instance, it is decidedly vertical, and contracts to the same extent as a cat’s. In some Water-snakes, and in Sea-snakes generally, the round pupil may contract to a mere dot. The contraction of the pupil is independent on the two sides.

The snout, or the part of the head anterior to the eyes, may be short or long, rounded or pointed, depressed or compressed, sometimes projecting strongly beyond the mouth, turned up at the end, or terminating in one (Langaha) or two (Herpeton) long scaly dermal appendages. In some burrowing forms it is provided with a more or less trenchant horizontal or vertical edge. When the sides of the snout (loreal region) form an angle with the upper surface, the angle is termed the “canthus rostralis,” which may be intensified by the loreal region being concave.

The deep pits which are sometimes present on the lips or between the nostril and the eye (loreal pit) will be alluded to further on under Sensory Organs.

The nostrils are either lateral, or, in the aquatic forms, directed upwards, sometimes entirely on the upper surface of the snout.