44 
TORTOISES AND TURTLES . 
SKELETON OF TORTOISE IN LONGITUDINAL SECTION. 
respectively articulated the arm-bone and thigli-bone, so that, as shown in the 
figure of the skeleton, these bones actually come within the ribs, instead of being 
external to them, as in all other living animals. At the fore and hinder extrem¬ 
ities of the shell are left large apertures, through which are protruded the head 
and neck, the fore and liind-limbs, and the tail. A large number of tortoises are 
able to retract both the 
head, limbs, and tail 
within the margins of 
the shell, the apertures 
of which are then filled 
up; such portions of the 
head and limbs as are 
exposed being protected 
by horny shields. 
With the exception 
of the marine leathery 
turtles and the fresh¬ 
water soft-tortoises, in 
which it is invested merely with a continuous leathery skin, the shell of 
Chelonians is covered with a number of horny plates, which, in the adult state at 
least, are in contact with one another by their edges. As these horny shields are 
very important in determining the different species of tortoises, it is essential to enter 
in some detail into their mode of arrangement, and the names by which they are 
known. In the carapace of any ordinary tortoise, such as the one represented in 
the left-hand figure at the head of the chapter, we shall find that the middle line 
of the back, exclusive of the margins, is occupied by a single row of large polygonal 
shields, symmetrical in themselves; these shields, which are marked v in the 
accompanying diagram, being known as the vertcbrals. On either side of this 
median series is another row of shields c, which are not symmetrical in themselves, 
and are termed costals. The extreme margins of the carapace are formed by a 
large series of much smaller shields, of which the anterior unpaired one (nu) is 
termed the nuchal, and the posterior ( ca ), which may be either single or double, 
the caudal. Between the nuchal and the caudal are a series, generally eleven in 
number on each side, designated marginals (in'). These same marginal shields, 
being angulated, pass over the edges of the middle portion of the shell, and thus 
cover the sides of the middle of the plastron, or lower shell, as shown in the right- 
hand figure of the accompanying diagram. The shields of the plastron proper are 
generally arranged in pairs, which may be termed, commencing anteriorly, gulars 
(gu), humerals (hit), pectorals (pc), abdominals (cib), femorals (fe), and anals (an). 
In some cases, as will be illustrated in the sequel, the two gulars may, however, be 
separated by a single intergular\ while, as in the accompanying diagram, there is 
frequently an inguinal shield immediately in advance of each notch for the 
hind-limbs. 
This disposes of the external horny shields; but a few words are necessary 
with regard to the bony elements constituting the shell of a tortoise. On stripping 
off these horny shields from the carapace of a tortoise, the underlying solid shell, 
