44 TORTOISES AND TURTLES, 



respectively articulated the arm-bone and thigh-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 hind-limbs, and the tail. A large number of tortoises are 



able to retract both the 

 head, limbs, arid 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 



tUTtlCS ailQ the 



SKELETON OP TORTOISE IN LONGITUDINAL SECTION. 



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, w r e 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 vertebrals. 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 (m). 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 (hu), pectorals (pc), abdominals (ab), 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, 



