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. Besembling 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, 
if 
INFERIOR, LATERAL, AND SUPERIOR VIEWS OF THE HEAD OF A SNAKE. 
r, rostral shield ; ff, anterior and posterior frontal do.; v. interparietal do.: s, supraocular do.; 0, parietal do.; 
nri 1 , nasal do.; l, loreal do.; a, preocular do.; p, postocular do.; uu, upper labial do.; tt\ temporal do.; m, mental 
do.; **, lower labial do ; cc, chin-shields.—After Gunther. 
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¬ 
parts 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 
