TORTOISES. 143 



being only a high-developed sternum. These organs are merely 

 portions of the skeleton, which, in place of being lodged in the 

 depths of the soft parts^ has become the superfices, which is only 

 covered by a thin dry skin. 



This numerous and highly-interesting order of Reptiles, called 

 Chelonia (from x* xd °vr), a tortoise), are also called Testudmata, from 

 testudo, the Latin name for a tortoise, from the double shield in 

 which the bodies of all, whether terrestrial, fresh-water, or marine, 

 are enclosed. 



The skeleton of the Tortoise is perhaps the most extraordinary 

 structure with which we are acquainted. This oddly-organised 

 animal when first seen strikes the beholder with astonishment. 

 The carapace and plastron, with their connecting plates, form a sort 

 of protecting box, in which the animal lives, its head and tail ex- 

 cepted. In the Land Turtles the head and feet, which are compara- 

 tively feelingless, can be withdrawn within the protecting armour. 

 The ribs and sternum are both placed quite on the exterior of 

 the body, so as to form a broad dorsal shield on the upper surface, 

 and an equally strong ventral plate ; between these, the limbs and 

 the head can be more or less completely retracted. Nevertheless, 

 the modifications in the arrangements of the elements by which 

 these changes are accomplished are of the simplest nature. In 

 the Common Tortoise, the vertebrae of the neck and tail being con- 

 nected together in the ordinary manner, the neck and caudal region 

 of the spine present their usual flexibility, but the dorsal vertebrse are 

 strangely distorted, the upper arch being disproportionately deve- 

 loped, while the bodies remain almost in a rudimentary state ; the 

 spinous processes of these vertebrse are flattened and converted into 

 broad osseous plates, which form a longitudinal series along the 

 centre of the back, and connected together by means of sutures. 

 The ribs are changed into broad flat bones firmly united to each 

 other by sutures, and also to the lateral margins of the spinous 

 processes of the vertebrae, so that they form together a single broad 

 plate ; the heads of the ribs are feebly developed, and the intervals 

 between them and the portions of the vertebrse rilled up with ligament. 

 The margin of the shield thus formed by the dorsal ribs is further 

 enlarged by a third set of flat bones fixed by sutures around the 

 whole circumference of the carapace. 



The plastron or ventral plate is made up of nine pieces, of which 

 eight are arranged in pairs ; but the ninth, which is always placed 

 between the four pieces composing the two anterior pairs, is single, 

 and occupies the mesial line. The bones of the shoulder and hip 



