In the first place, then, remember that snakes have a very large number of those sections or pieces forming the spine which we call vertebræ. We ourselves have only thirty-three of these little parts when we begin life, and twenty-six afterward; this difference in number being caused by the fact that five of the joints very soon unite into a bony mass at the lower end, which we call sacrum, while four more unite into another, which we call the coccyx. But some snakes have hundreds of these vertebræ. The boas, for example, have no less than three hundred and four! In the next place, remember that all these vertebræ are fastened together by what we call ball-and-socket joints. That is, there is a round knob at the back of each vertebra which fits into a socket in front of the vertebra behind it. This gives to the spine of a snake great strength, for a vertebra cannot be forced out of its place without breaking the vertebra behind it. And it also allows the spine to be curled and twisted about in almost any direction; so that a snake can easily coil up its body like a spring, or even tie it into a knot. Then, remember that a snake has a great many ribs. We have twelve pairs of these important bones, most of which are jointed to the breast-bone in front. But a snake may have as many as two hundred and fifty-two pairs of ribs, while it has no breast-bone at all; so that the tips of all the ribs are free. And every rib is fastened to a vertebra of the spine by a ball-and-socket joint, just like those which fasten the vertebræ themselves together. Besides this, there are no less than five separate sets of muscles connected with the ribs, so that the snake can move those bones about quite easily. It is really by means of its ribs that a snake is able to glide over the ground. If you were to look at the under side of a snake's body, you would see that the scales are quite different from those on the upper part. On the back and sides the scales are quite small, and are almost oval, or oblong; but on the abdomen they are very long and very narrow, and are set crosswise like the laths of a Venetian blind. [Illustration: CHARACTERISTIC FORMS AND MARKINGS OF AMERICAN BIRDS' EGGS SEA-FOWL:--13. Guillemot. 14. Tern. 21. Skimmer. WATER-FOWL:--9, 16. Ducks, WADERS. 7. Heron. 11. Gallinule. 12. Snowy Plover. 23. Stilt Sandpiper. 24. Ring Plover. GAME-BIRDS:--6. Partridge. 19. Ptarmigan. BIRDS OF PREY:--3. Owl. 17. Buzzard-hawk. 20. Falcon CUCKOOS:--8. Cuckoo. 10. Roadrunner. SONG-BIRDS:--1. Mockingbird. 2. Towhee Finch. 4. Sparrow. 5. Oriole. 15. Blackbird (grakle). 18. Flycatcher. 22. Rosbin (Thrush). 25. Woodhouse's Jay.]