The embryo is closely coiled up in a spiral. Just before birth it is distinguished by a large, convex head, with large, prominent eyes, and a comparatively short body, the scales and ventral shields being much shorter than later in life. The umbilicus is situated in the posterior part of the body, from six to ten times as far from the head as from the vent. Long after birth the umbilical slit remains visible, and affords a means of distinguishing very young snakes from older examples of smaller species. In oviparous species the embryo is provided with a very conspicuous egg-tooth, pointing forwards and projecting from the notch in the lower border of the rostral shield; this egg-tooth is much reduced, and sometimes very indistinct, in the ovoviviparous species. The function of the egg-tooth is to cut through the tough eggshell. This, after the young has left it, shows one or several slits in its anterior extremity, cut as clean as if with a sharp knife. The egg-tooth becomes loose soon after birth, and is shed within a few hours or a few days, sometimes even before birth in ovoviviparous species. Frequent cases have been observed of dicephalous embryos or young, which may live for a short time; there are even records of a three-headed snake, stated to have been seen at Lake Ontario, and of snakes with two heads and two tails. Unless prematurely born with a considerable mass of vitellus attached to the umbilicus, the young immediately after birth resent all interference, hissing, snapping, or puffing themselves up, after the manner of their parents. The first shedding of the outer coating of the epidermis follows soon after birth; not before then does the young take to food. No snake appears to be able to breed before it is four years old. Well-authenticated instances of different species interbreeding are unknown, but specimens intermediate between _Vipera berus_ and _V. aspis_, and between _V. berus_ and _V. ammodytes_, have been assumed, with much probability, to be hybrids. ------------------------------------------------------------------------ CHAPTER X HABITS Snakes may be grouped, according to their mode of life, in five principal categories, gradually merging into each other, or two of them not infrequently found combined in one and the same species. These categories are:--Ground-snakes, Sand-snakes, Burrowing-snakes, Tree-snakes, and Water-snakes. Ground-snakes may be defined as living above ground, and only occasionally climbing bushes or entering the water. Among European genera, _Coronella_ and _Vipera_ are perfect examples of this type, whilst _Coluber_ and _Zamenis_ approach the Tree-snakes in often ascending bushes, or even trees. Sand-snakes are adapted for living on loose sand, in which they seek concealment. Such are _Lytorhynchus_ and some _Psammophis_ among the Colubridæ, _Cerastes_ among the Viperidæ. _Eryx_ connects this category with the next.