TBIONYCHIDJE. 



The connexion between the carapace and plastron is called the 

 bridge. 



Fig. 4. Shell of Chelone mydas, with epidermal homy shields. 



A. Carapace B. Plastrou. 



a. Abdominal. h. Humeral. n. Nuchal. 



an. Anal. iff. Intergular. p. Pectoral. 



o. Costal. im. Inframarginal. sc. Supracaudal. 



/. Femoral. m. Marginal. v. Vertebral. 



g. Gular. 

 The anterior inframarginal is also termed axillary, and the posterior inguinal. 



Suborder I. THECOPHOEA. 



Superfam. A. TRIONYCHOIDEA. 



Neck bending by a sigmoid curve in a vertical plane ; cervical 

 vertebrae without welt-developed transverse processes ; articulation 

 between the last cervical and the first dorsal vertebra by the zyga- 

 pophyses only. Mandible with articulary concavity ; outer border 

 of tympanic cavity notched ; pterygoids not narrowed posteriorly, 

 separated from each other, the basisphenoid joining the palatines. 

 Pelvis not anchylosed to the carapace and plastron. Fourth digit 

 with four or more phalanges. Epiplastra separated from the 

 hyoplastra by the A-shaped entoplastron. Marginal bones absent 

 or forming an incomplete series, not connected with the ribs. 



Family I. TRIONYCHIOE. 



Carapace and plastron without epidermal shields, covered with a 

 soft skin. Jaws concealed under fleshy lips ; snout ending in a 

 proboscis. Head and neck completely retractile. Ear hidden. 

 Only the three inner digits clawed. Tail very short. 



