248 TEXT-BOOK OF ZOOLOGY 



formed by the expansion of the dorsal spines of the vertebrae (see Part I., 

 p. 4). They are accordingly termed neural plates. To their right and left 

 lies a row of larger plates, which are formed by the expansion of the ribs, 

 and are hence called costal plates. They are surrounded by a series of 

 smaller marginal plates, which are formed by ossification of the lower 

 skin, or derma. (Where have we previously met with dermal ossifica- 

 tions ?) Both the nuchal and the two pygal plates, which lie respectively 

 in front and behind the neural plates, are dermal ossifications, as are also 

 the plates composing the ventral shield or plastron. 



2. Above this bony armour lie a series of smaller and larger detach- 

 able horny plates, constituting the so-called tortoiseshell. They lie 

 adjacent to each other, like the slabs of a pavement, and are developed 

 from the upper skin, or epidermis. These plates, do not coincide with 

 the subjacent bony plates, but are so arranged as to cover the sutures of 

 the latter. These epidermal structures are not cast periodically, like 

 those of lizards and snakes, but fragments are detached from the 

 surface, and are renewed by growth from within outwards. 



3. The two portions, dorsal and ventral, of the bony armour are 

 firmly united laterally. In front and behind openings are left of 

 sufficient width to allow the head and limbs to be withdrawn into the 

 shell. In the case of the head this retraction is rendered possible owing 

 to the fact that the vertebras of the long neck are movably articulated, 

 and can be folded back on each other in the form of an S. In retracting 

 the limbs, the humerus and femur are drawn forwards, and the lower 

 arm and leg laid up close against them. The tail is laid on one side, 

 against one of the hind-legs. The armour of the tortoise furnishes it 

 with an excellent means of protection against — 



B. Its Enemies. 



Nevertheless, cats and falcons manage to get their claws through 

 the openings, and to pull out the fleshy parts. An additional protection 

 to the animal is — 



C. Its Colour. 



This completely resembles that of the muddy banks where, in a safe 

 hiding-place, the animal sleeps and suns itself during the day. The 

 horny plates of the carapace, as well as the skin of the neck and head 

 and the horny scales on the legs and tail, are of a blackish-green colour, 

 dotted and streaked with yellow. The plates of the plastron vary in 

 colour from yellow to blackish-brown. 



