4O REPTILES AND BIKDS. 



V.*-~Sea Snakes, distinguished by a strongly-compressed tail, and 

 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 of 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 Typhlopida, 

 TortricidcZ) Xenopeltidce, and Uropeltidcz, 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 is 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 



