314 CHELONIA CHAP. 



The guiding taxonomic characters are fully mentioned at the 

 head of the different families, and are mostly internal. The 

 following " key," adapted from Boulenger, and based upon ex- 

 ternal characters, is preferable for practical purposes. 



For the position and names of th e horny shields see Fig. 6 1 on 

 p. 315. 



Shell covered with horny shields. 



Digits distinct, with 5 or 4 claws. 



Pectoral shields separated from the marginals Ijv infra- 

 marginals. 



Tail long and crested. Plastron small and cruciform. 

 North America . . Chehjdridae, p. 338. 



Tail long, covered with rings of shields. 



Plastron large. Indo-China Platysternidae, p. 345. 

 Tail short. North and f Dermatemydidae, p. 341. 

 Central America . \Cinosternidae, p. 342. 

 Pectoral shields in contact with the marginals. 



Plastral shields 1 1 or 1 2, without an intergular. 

 Neck retractile in an S-shaped 



vertical curve . . Testudinidae, p. 345. 



Plastral shields 13, an intergular being present. 

 Neck bending sideways undvr f Chelydidaeyj). 399. 

 the shell . . yPelomedusidae, 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 i^addle-shaped. 



Shell composed of regular series of 



bony plates. Two claws . Garettochelydidae, p. 404. 



Shell composed of very many small plates arranged like 



mosaic. No cla\\s . . . Sphargidae, p. 333. 



The vertebrae ;ire, sometimes in the various regions of the 

 same individual, amplii-, opistho- or pro-coelous, or even biconvex. 

 Traces of the chorda remain longest in the middle of the centra, 

 lutercentra 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 



