GENERAL CHARACTERISTICS. 



43 



group from other living reptiles, it is necessary to add somewhat to this in order 

 to give a comprehensive definition. As regards the skull, this resembles that of 

 the crocodiles, in that the quadrate-bone, with which the lower jaw articulates, is 

 firmly wedged in among the adjacent bones, to which its relations are, however, 

 somewhat different. Unlike all crocodiles the jaws are, however, entirely devoid 

 of teeth, and are encased with horn, so as to form a cutting beak, which is invari- 

 ably short. A further peculiarity in the skull of a tortoise is to be found in the 

 presence of a greatly developed median spine (sup) 

 projecting backwards from the hinder region ; exter- 

 nally to which are a pair of shorter processes. In 

 other respects, the skull is extremely variable, the 

 sockets of the eyes being sometimes, as in the figure 

 on p. 47, surrounded by bone, while in other cases 

 they are open behind. Sometimes, moreover, the bony 

 roof behind the eye-socket in the figure on p. 47 may 

 be prolonged backwards so as to cover the whole 

 of the region marked par in the annexed figure. 

 There is an equal amount of variation in regard 

 to the position of the nostrils, which sometimes open 

 on the palate close behind the beak, while they may 

 be situated, as in living crocodiles, close to the hinder 

 extremity of the skull. A most important feature in 

 the structure of these animals is to be found in the 

 circumstance that the ribs have but a single head 

 apiece, and that the more anterior ones articulate at 



the junction between two of the vertebrae, so that one portion of the head is 

 applied to one vertebra and the other portion to the adjacent vertebra. This 

 forms an important distinction from the whole of the orders treated in the 

 preceding chapter, in all of which the anterior ribs are provided with two heads, 

 both of which articulate to the sides of one and the same vertebra. Passing on to 

 the consideration of the bony shell, we find this to consist of an upper portion 

 or carapace, shown in the figure at the commencement of the chapter, and of an 

 inferior portion, covering the lower aspect of the body, which is termed the 

 plastron. When this shell attains its fullest development, the upper and lower 

 moieties are completely connected together, as shown in the accompanying figure 

 of the skeleton of a land -tortoise; but in certain groups the two remain more 

 or less separate, and in some cases the lower shell is but very slightly developed. 

 Moreover, while the carapace is generally immovably welded to the vertebrae of 

 the back and the ribs, in the so-called leathery turtle it is separate from both. 

 In its fullest developed form, the shell consists of a series of bones articulating 

 with one another at their edges by finely denticulated sutures, and thus forming 

 a continuous whole, capable of increasing in size by growth at the edges of its 

 component elements. In the carapace, the bones forming the middle of the back 

 are formed by expansions growing from the spines of the vertebrae, while the large 

 lateral plates grow upon the ribs, from which they are inseparable. Within the 

 cavity thus formed are placed the bones of the shoulder and pelvis, to which are 



UPPER VIEW OP THE SKULL OF THE 

 SOFT-TORTOISE OF THE GANGES. 



