OPOTERODONTA COLUBRIFORMIA. 211 



run in a certain sense on the extreme points of their ribs, which are 

 attached to dermal scutes ; for they move by alternately pushing the 

 ribs forward, and drawing after them the ventral scutes, which are 

 attached to one another and to the ribs by muscles. 



The Snakes feed exclusively on living animals, both warm-blooded 

 and cold-blooded, which they attack suddenly, kill and swallow whole 

 without mastication. Swallowing is effected thus : the teeth on the 

 jaws are alternately hooked further and further forwards into the 

 body of the prey, as a result of which the mouth and pharynx of 

 the snake are in a sense gradually drawn over the animal, whose 

 surface is at the same time made slippery by the abundant secretion 

 of the salivary glands. During this process the larynx is projected 

 forward between the rami of the jaws, so that respiration can be 

 maintained. After the completion of this laborious operation of 

 swallowing, the animal appears entirely prostrated, and passes a 

 long period in inactivity, during which the very slow but complete 

 digestion takes place. 



Snakes copulate, and are for the most part oviparous. They lay 

 a small number of large eggs, in which the embryonic development 

 may be already far advanced. There are, however, also viviparous 

 Snakes ; for example, the sea-snakes (Hydrophidce) and the vipers 

 (Pelias berus). 



Most of the species distinguished by size and beauty of colour 

 belong to the warmer zones, only the smaller forms extend into 

 northern temperate climates. Many Snakes are fond of the water 

 and are truly amphibious. Others live for the most part on trees 

 and shrubs, or on sandy ground ; others exclusively in the sea. In 

 the temperate countries they fall into a kind of winter sleep, in the 

 hot countries they undergo a summer sleep in the dry season. 



Sub-order 1. Opoterodonta. With narrow, non-distensible, slit- 

 like mouth, and immovably connected facial bones, without or with 

 only a short tail. They have solid hooked teeth only in the upper 

 jaw or in the lower jaw. Posterior limbs present as rudiments. 

 They live beneath stones, or in passages in the earth, and feed on 

 insects. 



Fam. Typhlopidae. Typhlopg lumlricalis Merr. (fig. 638), Antilles. T. ver- 

 micularis L., Greece. Stetiostoma nigricans Dum. Eibr., South Africa. 



Sub-order 2. Colubriformia. Both jaws armed with solid hooked 

 teeth. In the upper jaw the last tooth may be grooved, and then 

 may be either without poison glands, or may be connected with the 



