DISTINCTIVE CHARACTERS. 



175 



glass covers the face of a watch. When a snake changes its skin, which it does 

 several times during the year, the discs over the eyes peel off with the rest, and 

 appear as lenses in the dry slough. Equally characteristic is the absence of any 

 external aperture of the ear ; a peculiarity which has given rise to the scriptural 

 expression of the deaf adder stopping its ears. Resembling that of lizards in its 

 flattened form, the tongue of snakes is narrow and smooth, and terminates in a 

 fork formed by two long thread-like points, while at its base it is inserted into a 

 sheath from which it can be protruded at will. The head, although not very large, 

 is generally wider than the body, from which, as already said, it is but seldom 

 separated by a recognisable neck, and is usually oval or triangular in shape, with 

 a more or less well-marked depression. Near the sides of its extremity, and some- 

 times at the very tip, are situated the nostrils ; while posteriorly the gape in some 

 cases extends almost to the back of the head. Superiorly, as well as on its under- 

 surface, the head is generally covered with a number of large symmetrical shields, 



INFERIOR, LATERAL, AND SUPERIOR VIEWS OK THE HEAD OF A SNAKE. 



r, rostral shield ; ff, anterior and posterior frontal do. ; v, interparietal do. : s, supraocular do. ; 0, parietal do. ; 

 mi 1 , nasal do. ; I, loreal do. ; a, preocular do. ; p, postocular do. ; uu, upper labial do. ; tf 1 , temporal do. ; on, mental 

 do.; **, lower labial do ; cc, chin-shields. — After Giinther. 



having their edges in apposition, and varying in relative size in the different 

 groups. Although the blind snakes have a uniform cuirass of polished scales all 

 round the body, while some of the sea-snakes also have the scales of the under- 

 pays similar to those of the back, in the great majority of the order the under 

 surface of the body is protected by large transverse shields, extending completely 

 across it from side to side. These broad shields often extend as far backwards as 

 the termination of the body proper ; while at the commencement of the tail, and 

 thence backwards to the extremity, they are replaced by a double row. These 

 large inferior shields take an important part in the progression of snakes on land, 

 and hence we see why they are wanting in the marine forms. 



In all snakes the number of joints in the backbone is very large ; and each 

 of these, with the exception of a few near the extremity of the tail, is provided 

 with a pair of rather long, slender, and curved ribs, the extremities of which 

 correspond to the large inferior shields of the body in the species where these 

 are present. Superiorly the ribs, as shown in the figure on p. 104, articulate by a 



