40 REPTILES AND BIRDS. 
V.—Sea Snakes, distinguished by a strongly-compressed tail, ana 
by the position of the nostrils, which are placed as in the last group. 
They live in the sea, only occasionally approaching the land, feed 
on marine fishes, are viviparous and venomous. One genus only 
(Platurus) has the ventral shields so much developed as to be able 
to move on land. No oceanic serpent is known of gigantic dimen- 
sions, such as is currently alleged to have been seen by unscientific 
observers. 
** Although these five groups,” remarks Dr. Giinther, “are not 
separated from each other by defined lines of demarcation, and 
frequently pass into one another by intermediate forms, yet a family 
and genus which should be composed of species of several ot these 
groups would be a very unnatural assemblage of heterogeneous 
forms.” 
It is also remarked by the same naturalist that there is no sharp 
boundary line between the order of Snakes and that of Lizards. 
There are various limbless Saurians of Ophidian appearance, but 
the systematic position is decided by the structure of their jaws. 
The Common Orvet, or Slow-worm, is a familiar instance. On the 
other hand, certain Ophidians remind us, by several characteristics, of 
the Saurian type—as the Snakes constituting the families ZypAlopide, 
Tortricidae, Xenopeltide, and Uropeltide, which are distinguished. by 
polished, closely adherent, rounded, sub-equal scales, much resem- 
bling the smooth scales of various Scincoid Lizards ; most of them 
have a very narrow mouth, unlike the enormous gape of the typical 
Serpents ; and some are without that longitudinal fold in the median 
line of the chin which jis so characteristic of most Ophidians ; more- 
over, most of them have rudiments of the bones of a pelvic arch. 
“The reason,” alleges Dr. Giinther, “‘ why we adopt the view of those 
systematists who refer such Reptiles to the Ophidians, instead of 
associating them with the limbless Scincoid Lizards, is the loose 
connection of the jaw-bones, a character which must be considered 
as peculiar to the Ophidians, and which is only somewhat less 
developed in the families mentioned than in the typical forms; the 
two halves of the lower jaw in Ophidians, namely, are not united by 
a bony symphysis, but by an elastic ligament. The peculiar mobility 
of the jaw bones enables the Snakes to extend the gape in an extra- 
ordinary degree, and to work their prey down through the collapsed 
pharynx.” 
The same naturalist classifies the Ophidia into three sub-orders, 
in which the venomous snakes are separated from the others; but 
