CHELONID^E. 25 



Section II. CRYPTODIRA. 



Shell with the plastron unconnected with the pelvis ; generally- 

 covered with epidermal shields ; entoplastral (when present) oval, 

 rhomboidal, or T-shaped, and not separating the epi- from the hyo- 

 plastral ; a full series of marginals articulating with the ribs. 

 Mesoplastral absent in all known forms (Pleurosternum and its allies 

 being referred to a distinct section). Skull with quadrate articu- 

 lating with a concavity in mandible ; posterior border of tympanic 

 ring with distinct notch ; pterygoids in contact, narrow, and late- 

 rally emarginate, and extending forwards to join the vomer, which 

 divides the palatines. Cervical vertebrae of moderate length, with 

 rudimentary or no transverse processes ; sacral and caudal ribs 

 articulating both with arches and centra of vertebrae. Not more 

 than three phalangeals in the digits. Head retracted by curvature 

 of the neck in a vertical plane. 



The ossification of the shell may or may not be complete, and 

 the plastron may in some cases be free from the carapace ; occasion 

 ally the shell may be sculptured, and in some instances may also 

 lose its epidermal shields. In the great majority of cases an inter- 

 gular shield is wanting in the plastron. Very generally, moreover, 

 the neural and suprapygal bones form a complete series, although 

 there are numerous exceptions. The humerus is subject to great 

 variation in form. For details as to the structure of the skull and 

 shell see Boulenger, ' Catalogue of Chelonians, &c./ pp. 11-18. 



Family CHELONID^. 



Ossification of shell more or less incomplete ; plastron distinct 

 from carapace, retaining, at least for a long period, larger or smaller 

 vacuities, and with digitate lateral extremities ; nuchal without 

 costiform processes ; carapace frequently cordiform, and in some 

 cases with the antero- and postero-lateral surfaces of the hexagonal 

 neurals subequal. Caudal vertebras proccelous ; cervicals very 

 short. Skull with temporal fossa completely roofed over ; squamosal 

 joining parietal, and the latter articulating by a very long suture 

 with postfrontal ; quadratojugal entering into formation of tympanic 

 ring ; tympanic cavity completely open posteriorly ; bones of palate 

 developing palatal plates to floor the narial passage. Scapula and 

 precoracoid forming a very obtuse angle at their junction ; humerus 

 (fig. 6) more or less flattened, with the axis of the head generally 

 more or less nearly coincident with that of the shaft, and the radial 

 process relatively small and placed entirely, or to a great extent, 



