46 CLASS REPTILIA. 



that portion which intervenes between the two divisions of 

 the armature. This skin is usually covered with scales, or 

 with scaly turbercles more or less inter-approximated. Its 

 contexture is so solid that the sharpest instruments can with 

 difficulty make any entrance into it. It is closely fixed 

 through its entire border, at some distance from the internal 

 edge of the carapace and plastron, and it is susceptible of very 

 great extension. 



The head of tortoises has, in general, the form of a quad- 

 rangular truncated pyramid, that is, it presents four faces, 

 more or less convergent, and more or less rounded, the upper 

 one of which is formed by the cranium, the lateral by the 

 orbit of the eyes, and the upper jaw, and the under one by 

 the under jaw. It presents, besides, an anterior truncation, 

 where the nostrils are situated. 



The eyes of tortoises are generally small, though usually 

 placed in very large orbits. They are conformed like those 

 of other reptiles, but independently of the two external and 

 horizontal lids, there is a third lower and vertical. The 

 lower is more susceptible of elevation than the upper is of 

 depression. It would appear that the sense of sight is not 

 much extended in those animals, and that of hearing still less 

 so ; but it cannot be true that they are deaf, for they have a 

 tympanum, though concealed, which may yet be remarked 

 from the tension and colour of the skin in that peculiar place. 



The nose of tortoises is formed by two oblong holes, which 

 are seen in the upper part of the muzzle. Some species have 

 them placed at the end of a short and cylindrical trunk, 

 which projects from this same part. It has also been asserted 

 that the sense dependent on this organ is also very feeble in 

 the chelonia ; but direct observation does not authenticate 

 this conclusion. 



The tortoises have a slender pyramidical tongue, of thrice 

 greater length than breadth, covered with a great number of 



