2 34 SNAKES. 



scales in most of the species. In land snakes we saw how 

 admirably adapted are the broad, ventral plates 

 for assisting those reptiles over rough sur- 

 faces, as affording hold ; but the HydrophidcB 

 requiring no such aid in a fluid, those scutse 

 would be useless ; they are therefore, excepting 

 in one or two species, entirely absent, or but 

 slightly developed, and the belly -is ridged 

 instead, like the keel of a boat. ^ . . . 



' Portion of the 



The nostrils are small, placed horizontally on s^a^nakl? above 



1 r ^ ' ^ TT J ' 1 i ^"^^ below the 



the top of the snout, as m the nomalopsiacs, and anus, with no dis- 

 tinction in tail 



in most of the sea snakes they are contiguous. ^^^'^^• 

 They are, moreover, furnished with a valve, which is under 

 control of the will, opening to admit air, and closing to 

 exclude water when diving. For, be it remembered, these 

 marine reptiles breathe through their nostrils even more 

 entirely than terrestrial snakes, the latter being better able 

 to indulge their yawning propensities, or to occasionally 

 respire slightly, and through parted lips and the tongue chink 

 as well. Sea snakes, on the contrary, not requiring the 

 continual use of their tongue to feel and explore surround- 

 ings, and not using it below water, are not provided with 

 the little centre chink for its exsertion ; but the middle plate 

 of the upper lip, i.e. the 'rostral shield ' (see illus. p. 238), is 

 altogether of a different form. Indeed, the centre plates or 

 shields in both lips are conspicuously modified, the upper 

 one often inclining downwards in a point which fits into 

 the lower one shaped to receive it, so that the mouth is 

 firmly closed to keep out the water. Less required, the 

 tongue is shorter and less developed, the tips are less hair- 



