Before entering upon a description of the greatly feared though harmless Grass, Ringed, or Common Snake, it would be well to say a few words on the structure of Snakes in general, and so avoid some amount of repetition, for in a general way our three species are alike. The Slow-worm, our legless Lizard, affords a convenient transition to the Snakes; but the bony skeletons of Snake and Slow-worm exhibit considerable differences. No Snake possesses a breastbone, bladebone, or collarbone, so that all the ribs are free at their ends, and they are strongly curved to produce the cylindrical form of body. When bulky food is taken the ribs can be flattened out to allow of the necessary distension of the body until digestion and muscular pressure have reduced the bulk. The bones of the skull are connected so loosely that the head can be flattened and widened, so that the mouth can admit prey equal to three times the size of the Snake's head under normal conditions. To assist in the swallowing of such large bodies, the two halves of the lower jaw have no bony connection but are united instead by elastic ligaments, so that each half can be moved independently of the other, and by the alternate movement of the two sides with the teeth all pointing backwards the food is worked back to the throat. There are other teeth on the roof of the mouth which make it difficult for living prey to struggle forward and escape when once it has been seized. The teeth are all thinly coated with enamel, and are not planted in sockets. If they should get broken by the severe work imposed upon them, they are soon replaced by others which lie in reserve. Poison fangs are much larger than ordinary teeth, and the enamel is folded so as to produce a groove down which poison is pressed from a gland into the wound made by the point of the fang. The fang is hinged at its base and ordinarily lies pressed back upon the upper jaw, and is only "erected" when the Snake is prepared to strike. [Illustration: *The Head Shields of a Snake.* _r_, rostral shield; _ff_, anterior and posterior frontal; _v_, interparietal; _s_, supraocular; _o_, parietal; _nn'_, nasal; _l_, loreal; _a_, preocular; _p_, postocular; _uu_, upper labial; _tt'_, temporal; _m_, mental; **, lower labial; _cc_, chin-shields.--After Günther.] Externally the Snake is covered by small overlapping scales on the upper parts and by broad plates on the under surface. The head is covered mainly by shields, each of which has a definite name, but for the purposes of this book it is not necessary to enter upon a tedious recital of these terms, beyond giving them for reference under the diagram of a Snake's head. [Illustration: *Skeleton of Snake.*]