69 MANUAL OF NATURAL HISTORY. 



bers are reversed, three being external and two in- 

 ternal, so that their whole number of twenty toes 

 may be looked on as arranged in four longitudinal 

 series, each containing five. 



II. ORDER.— Serpents (Ophidia). 



Mouth dilatable ; facial bones moveable ; jaws 

 toothed ; lower jaw bones united by ligaments - in 

 in front ; eyes without eyelids ; external ears none ; 

 tongue very long, retractile into a sheath at its base, 

 apex forked, very long, slender, tapering; limbs none, 

 or only rudimentary ; one lung very large, the other 

 very small or rudimentary. 



I. SUB-ORDER. — Venomous-Serpents (Yiperina). 



: Jaws weak, upper one toothless, with distinct poi- 

 son-fangs in front, lower toothed ; head large behind ; 

 crown with scales, rarely with shields ; hinder limbs 

 not developed ; eyes lateral ; nostrils apical, lateral. 



1. Family. — Rattle-Snakes (Crotalidse). Face with 



a large pit on each side, between eye and 

 nostril ; crown covered with scales or small 

 shields ; upper jaw with long fangs ; belly- 

 shields broad, band-like ; anal spurs none; 

 viviparous. 



2. Family. — Vipers (Viparidse). Face without sub- 



orbital pits ; rostral shields broad, band-like ; 

 scales mostly keeled ; tail short, tapering. 



II. SUB-ORDER.— Water-Serpents (Hydrina). 

 Jaws strong, toothed ; fangs moderate, intermixed 



