THE TORTOISES 



2401 



SKELETON OF TORTOISE IN LONGITUDINAL SECTION. 



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 extremites 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, 

 and tail within the mar- 

 gins of the shell, the 

 apertures of which are 

 then filled up; such por- 

 tions of the head and 

 limbs as are exposed be- 

 ing 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 following diagram, be- 

 ing 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 ex- 

 treme margins of the carapace are formed by a large series of much smaller shields, 

 of which the anterior, unpaired one (mi) is termed the nuchal, and the posterior (ca), 

 which may be either single or double, the caudal. Between the nuchal and the cau- 

 dal 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 following diagram. The shields 

 of the plastron proper are generally arranged in pairs, which 'may be termed, com- 

 mencing anteriorly, gulars (gu}, humerals (hit], 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 is 

 shown in the 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, 

 as shown in the right-hand figure at the head of the chapter, will be seen to be 



