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 
Ckelonia (from yeadvn, a tortoise), are also called Zestudinata, 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tively feelingless, can be withdrawn within the protecting armour.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 vertebrz 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 vertebree are 
strangely distorted, the upper arch being disproportionately deve- 
loped, while the bodies remain almost in a rudimentary state; the 
spinous processes of these vertebrze 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 vertebrze, 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 vertebre filled 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 
