OPHIDIA. 5 



cavity in front of the succeeding vertebra. This structure of ball and socket 

 allows of free lateral motion; but the spinous processes of the back prevent 

 motion, up and down, to any great extent. The whole body is covered 

 above with scales, mostly small and imbricated, and below with large 

 quadrangular plates. 



6. The trachea is very long, and surrounded by simple, complete, cartilaginous 



rings only at its anterior half; the lung is single, and extends nearly the 

 whole length of the body. In some is found a rudiment of a second lung. 



7. The heart is placed far back, and is provided with two auricles and a single 



ventricle. 



8. Serpents are for the most part oviparous — the eggs are covered with a 



calcareous flexible shell — the young burst from them into life with the same 

 form they are permanently to retain. There are some, however, among the 

 poisonous serpents that are viviparous. 



So far the true serpents agree in general in their characters and organization; 

 but in some is developed an entirely new set of organs, which invest their 

 possessors with singular and noxious properties. 



This peculiarity of structure leads to a very natural division of the serpent 

 tribe into two great sections — venomous and non-venomous serpents. 



Venomous serpents are again distributed into different families, according to 

 the arrangement of their fangs, dilatability of jaws, pits about the head, &c. &c. 



