Shell covered with horny shields. Digits distinct, with 5 or 4 claws. Pectoral shields separated from the marginals by inframarginals. Tail long and crested. Plastron small and cruciform. North America _Chelydridae_, p. 338. Tail long, covered with rings of shields. Plastron large. Indo-China _Platysternidae_, p. 345. Tail short. North and { _Dermatemydidae_, p. 341. Central America { _Cinosternidae_, p. 342. Pectoral shields in contact with the marginals. Plastral shields 11 or 12, without an intergular. Neck retractile in an p. S-shaped vertical curve _Testudinidae_, p. 345. Plastral shields 13, an intergular being present. Neck bending sideways under the shell { _Chelydidae_, p. 399. { _Pelomedusidae_, p. 390. Limbs paddle-shaped, with one or two claws _Chelonidae_, p. 378. Shell without horny shields, covered with soft, leathery skin. Digits distinct, broadly webbed, but with only three claws _Trionychoidea_, p. 404. Limbs paddle-shaped. Shell composed of regular series of bony plates. Two claws _Carettochelydidae_, p. 404. Shell composed of very many small plates arranged like mosaic. No claws _Sphargidae_, p. 333. The VERTEBRAE are, sometimes in the various regions of the same individual, amphi-, opistho- or pro-coelous, or even biconvex. Traces of the chorda remain longest in the middle of the centra. Intercentra occur regularly on the first two or three cervicals, and then again in the tail as paired or unpaired nodules, or as short chevrons. The latter occasionally fuse with the caudal end of their centra. Intercentral discs of fibrous cartilage occur regularly in the neck and tail. The ribs develop originally in the same transverse level with these discs, and frequently the anterior thoracic vertebrae retain this intercentral or intervertebral position throughout life. Farther back they often show a gradual change from the intercentral to a more central and ultimately {315}remarkable to a purely neural attachment. In all the Chelonia the ribs are devoid of the tubercular portion.