142 ; GUIDE TO THE 
of Water-snakes as this term is specially restricted to 
two groups of snakes of very different structure, one 
section constituting the true Fresh-water Snakes or 
Homalopsidce next to be mentioned. They are dis¬ 
tinguished from the Land Colubrine snakes by having 
the nostrils placed on the top of the head and provided 
with a valvule by which they can be closed at will. 
The position of the nostrils has reference to their aquatic 
habit, because snakes, whether terrestrial or aquatic, always 
breathe atmospheric air, although they have generally only 
one lung. Some of these Fresh-water snakes, such as the 
Hypsirhina enhydris , extend their range even to the sea, 
and one of them, apparently peculiar to Siam, has two 
long feelers on the snout. Somewhat allied to these snakes 
is the very peculiar Egg-eating snake of South Africa, 
in which the eggs, before being swallowed, are broken by a 
series of throat or gular teeth. The snake is provided with 
small teeth in its mouth, but they do not prevent the 
passage of eggs, and they are so feeble, that the shells 
are not broken, but the egg passes down uninjured as 
far as the gular or throat teeth which retain it, and the 
muscles of the throat contracting around the egg, it is 
cut by these teeth,- and no sooner is this done, than the 
mouth opens and the egg-shell is rejected, its fluid con¬ 
tents passing down into the stomach. Were the eggs 
broken in the mouth the serpent would lose its only nourish¬ 
ment. These throat teeth are very remarkable from the 
circumstance that they are not teeth in the strict sense of 
the word, but processes from the under surfaces of the 
throat vertebrae, their ends projecting into the gullet and 
covered with a thin layer of enamel. This snake constitutes 
the Family of Throat-toothed serpents. Besides these there 
