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 The Skeleton (Fig. 267).—The carapace is covered with thin epidermal plates which belong to the skin. The bony nature of the carapace is seen when the plates are removed, or if its inner surface is viewed (Fig. 267). It is seen to consist largely of wide ribs (how many?) much flattened and grown together at their edges. The ribs are seen to be rigidly attached to the vertebræ. The rear projections of the vertebræ are flattened into a series of bony plates which take the place of the sharp ridge found along the backs of most vertebrates. Show that the shell of a turtle is not homologous with the shells of mollusks. Does the turtle have shoulder blades and collar bones? Hip bones? Thigh bones? Shin bone (fibia) and splint bone (fibula)? (Fig. 267.)

C, rib plates; M, marginal plates; B, plastron; H, humerus bone; R, radius; U, ulna; Fe, femur.

(tortoise).

a, veins; b, f, right and left auricles; cg, ventricle; d, arteries to lungs; e, veins from lungs; i, n, two branches of aorta. Compare with Fig 269 and colored Fig. 2.

Do the plates formed by the ribs extend to the edge of the carapace? See Fig. 267. About how many bony plates form the carapace? The plastron? Do the horny plates outside correspond to the bony plates of the shell?