These observations of Sir Everard Home apply to all Snakes; but the muscles were compared with a skeleton of the _Boa-constrictor_ in the Hunterian Museum, which is thirteen feet nine inches in length. The habit of attaching themselves to trees, and holding on by the tail, their heads and bodies floating listlessly on some sedgy river, is explained by the structure of the tail. Dr. Meyer has minutely described the manner in which they hook themselves on to a tree, which gives them the power of a double fulcrum. The apparatus which gives this power is a spur or nail on each side of the vent in the _Pythonidæ_, in which the anatomist discovered the elements of an unguinal phalanx articulated with another bone much stronger, which is concealed under the skin. Following the arrangement of the _Pythonidæ_, adopted by Dr. J. E. Gray in the Catalogue of the British Museum, we find:-- I. _Morelia_, having a strong prehensile tail, distinct head, truncate muzzle, crown of the head with small shield-like plates. Of this genus there are two species. The Diamond Snake (_M. spilotes_), a native of Australia, and of a bluish-black colour; and the Carpet Snake (_M. variegata_), from Port Essington and Swan River. It is whitish, with irregular black-edged olive spots, and an olive head, with two or three white spots in the centre of the crown. II. _Python_, having the crown shielded to behind the eyes. Of this genus there are two species, which have sometimes been referred to the Boas. The Pythons bear the same general appearance. Upon their bodies is traced a sort of blackish-brown chain, presenting nearly quadrangular links upon a clear yellowish ground, extending from the nape of the neck to the extremity of the tail. The suscephalous region is partly covered by a large brownish-black spot. Upon each side of the head is a black band, which frequently extends from the nostril, passing by the eye, as far as, and up to, the commissures or corners of the lips. [Illustration: Fig. 13.--Natal Rock Snake (_Hortulia natalensis_).]