TURTLES. 



441 



armature produced by a secondary growth, the liardening of certain portions of the 

 dermal covering. In all cases the head, tail, and limbs are capable of being pro- 

 truded from between the margins of the thus formed plates, and may be modified for 

 special habits ; as broad and fin-like for marine, simply webbed for inland aquatic, or 

 short and stump-like for terrestrial life. Four limlis are always present ; and the jaws 

 are unprovided with teeth proper, being simply encased in hom, like those of birds. 



As to the shield-like covering, it is of considerable importance to the systematic 

 zoologist, often presenting most distinctive characters. By reference to the illustra- 

 tion it will be seen to consist of two portions, the dorsal, or carapa.v, and the ventral, 

 or plastron, and to be divided into geometrically shaped portions, or plates of a horny 

 nature, which in no way follow the outline of the underlying bones. These plates are 



Fig. 255 — Diagram illustrating the dorsal and ventral plates of a turtle ; ab, abdominal ; an. anal ; c, caudal ; cos, 

 costal ; (7, gular ; i, inguinal ; m, marginal ; n, nuchal ; ;), pectoral ; pa, pre-anal ; p<j, post-gular ; v, vertebral ; 

 X, axillary. 



of considerable commercial value in some turtles ( Chelonia) as they are the ' tortoise- 

 shell ' of commerce. 



From the rigid jiortion of the vertebral column, the expanded processes of which, 

 together with the suturally united ribs, form tlie carapax, the flexible neck jirojects, 

 and is capable of being more or less com])letely retracted into the cavity of the shell. 

 The eight vertebra? of which it consists are entirely destitute of ribs. Posteriorly, the 

 sacral portion of the column is bent down, free from the carapax, and terminated by 

 a flexible tail, which is, on the occasion of danger, not withdrawn \iiider the shield, 

 but simply bent round against the side of the body. 



The bones of the head are firmly united to each other, the skull resembling that 

 of birds, though the cranial capsule is considerably less in size. The orbits arc sepa- 



