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 OF THE SKULL OF THE 
SOFT-TORTOISE OF THE GANGES. 
