That the skin of many snakes contains soluble colouring matter of a special kind is well known, green snakes, such as _Dryophis prasinus_ and _Lachesis gramineus_ staining the spirit in which they are preserved. Chemists have not yet paid attention to this question, which requires investigation. Melanism is frequent in snakes, and sometimes affects all individuals in the same locality. It seems undesirable to bestow varietal names on such aberrations, as is so frequently done by systematists, any more than we should in the case of albinos. Melanism may be produced in two ways: by an extension of the black markings, which invade the whole surface, as in the males of _Vipera berus_; or by a general darkening of the ground colour and of the markings, as in the females of the same species. In the latter case, the markings reappear under certain lights or after a prolonged sojourn in spirits. Sometimes, as in _Zamenis gemonensis_, the uniform black colour appears only as the snake approaches the adult condition, the young having the normal livery. Partial albinism is rare; perfect albinism, characterized by absence of black pigment in the eye, rarer still. Cases have been observed, among European species, in _Tropidonotus natrix_ and _tessellatus_, in _Coluber longissimus_, and in _Coronella austriaca_. ------------------------------------------------------------------------ CHAPTER IV SKELETON The typical Ophidian skull is characterized by a solidly ossified brain-case, with the distinct frontals and the united parietals extending downwards to the basisphenoid, which is large and produced forward into a rostrum extending to the ethmoidal region. The nasal region is less completely ossified, and the paired nasals are often attached only at their base. The occipital condyle is either trilobate and formed by the basioccipital and the exoccipitals, or a simple knob formed by the basioccipital; the supraoccipital is excluded from the foramen magnum. The basioccipital may bear a strong, curved ventral process or hypapophysis (in the Vipers). The prefrontal is situated, on each side, between the frontal and the maxillary, and may or may not be in contact with the nasal; the postfrontal, usually present, borders the orbit behind, rarely also above, and in the Pythons a supraorbital is intercalated between it and the prefrontal. [Illustration: FIG. 3--SKULL OF _Python amethystinus_. (From British Museum Catalogue of Snakes)