2400 



TORTOISES, TURTLES, AND PLESIOSAURS 



J u 



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, some- 

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

 teeth, and are incased with horn, so as to form a cutting beak, which is invariably 

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



of a greatly-developed median spine (.?/>) projecting 

 backward from the hinder region; externally 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. 2404, sur- 

 rounded by bone, while in other cases they are open 

 behind. Sometimes, moreover, the bony roof be- 

 hind the eye socket in same the figure may be 

 prolonged backward 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 rib8 haVe bUt * ^^ hfiad ^^ Md th&t 



the more anterior ones articulate at the junction be- 

 tween two of the vertebrae, so that one portion of the head is applied to one verte- 

 bra 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 as- 

 pect of the body, which is termed the plastron. When this shell attains its fullest 

 development, the upper and lower moieties are completely connected, as shown 

 in the following 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 vertebras 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 

 respectively articulated the arm bone and thigh bone, so that, as shown in the figure 



UPPER VIEW OF THE SKULI, OF THE 

 SOFT TORTOISE OF THE GANGES. 



