XVIII INTRODUCTION. 



at the same instant. Each step is short, hardly equal the length of one of 

 the ventral scutes. The ribs are attached to the scute in such a way that 

 its free posterior edge, braced against projections on the surface over which 

 the animal is moving, serves as a .support from which the body is pushed 

 ahead. A track left by a gliding snake may be a right line. This motion 

 can he executed so that the entire body moves along without apparent effort. 

 In walking opposed ribs are moved forward alternately, and the edge of the 

 end of the scute under the rib moved holds what is gained by the motion 

 while the opposite extremity is stepped ahead of it. This mode of travel- 

 ing gives rise to flexures of the body, and the track is undulating. I3v the 

 third method, the creature pushes or elbows its way with the body ami tail, 

 as do the eels or snake-like batrachians. Mostof the worm snakes progress 

 in this way. In fact, any snake hurriedly moving among grass, hushes, or 

 rocks, takes advantage of any support he can get for his sides or extremity. 

 Except when gliding, serpents in motion commonly use the three methods 

 at the same time. 



The outer dermal layers are generally folded into the semblance of scales ; 

 occasionally they are tubercular or granular. The scaly folds are most often 

 imbricated on the body, and occasionally over the head. Generally labial 

 and other plates of the head are non-imbricated. When the epiderm is to 

 be shed, the snake rubs it loose at the lips, and, creeping against some 

 object, manages to strip it off entire, and ordinarily inside out. The num- 

 ber of times the skin is shed in a season is variously stated by different 

 authorities. .Serpents that were kept for study have sloughed hut once in a 

 year, and I am inclined to believe this is the common practice. Nocturnal 

 serpents are few, and belong to the torrid zone. Most, if not all, are affected 

 by a period of comparative inaction; during the winter this takes the form 

 of hibernation or a winter sleep. Such a sleep or rest is not an absolute 

 necessity; when waked and kept lively everyday of the winter they do not 

 seem to be any the worse for it in the spring. 



In coloration these reptiles are excessively varied; individual variation is 

 great in the same species; the colors are more bleached in exposed situa- 

 tions; and of terrestrial species, those portions of the body that are habitu- 

 ally raised from the ground are commonly whiter beneath than the balance 

 of the ventral surface. 



Certain good authorities have adopted the idea that some snakes swallow 

 their young for protection in times of danger. The evidence hardly appears 



