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 be 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. By the 
third method, the creature pushes or elbows its way with the body and tail, 
as do the eels or snake-like batrachians. Most of the worm snakes progress 
in this way. In fact, any snake hurriedly moving among grass, bushes, 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 
imbriecated 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 but 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 every day 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 
ereat 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 
