262 ZOOLOGY. 
occipital foramen, there is a single one placed below, formed either by the 
basi-occipital, or by this together with part of the ex-occipitals, as in the 
Chelonia. The ribs are always present, sometimes in great development 
and number. The vertebre exhibit a great diversity in the shape of their 
articulating faces, and are generally quite complicated in structure. The 
entire system exhibits a higher state of development, ossification is more 
complete, the apparatus of sense more perfect, this being well exhibited in 
the tympanic apparatus. Respiration is carried on by means of lungs. 
The eggs are generally protected by a caleareous or toughly membranous 
opaque covering ; and the development of the embryo is complicated by 
the presence of an allantois, and the amniotic sac with its peculiar liquid, 
the liquor amnii. 2 , 
Orper 2. OPpHIDIA. 
The Ophidia, or Serpents, are especially characterized among Reptilia by 
the limbs being either entirely absent, as in the majority of the species, or 
else being so rudimentary as to be discoverable only on dissection or very 
close examination. The bones of the face, excepting in a few species, 
possess a great deal of mobility. The lower jaw, instead of a direct articu- 
lation with the upper, is brought into connexion with it through the 
medium of two bones, movable on each other. The extremities of the 
lower jaw, also, instead of being anchylosed, are united by an extensile 
ligament. The mouth is variously provided with teeth, these in some 
species serving as a tube for the injection of a peculiar poison secreted by 
special glands. There are no movable eyelids, nor is there any external 
ear. The vertebre are very numerous, always exceeding 100, the pos- 
terior articulation spherically convex, the anterior spherically concave. 
Ribs, very numerous, free. The skin is covered with scales of various 
shapes and proportion, an epithelium from which is shed once or several 
times in a year, usually in one entire piece. The tongue is soft, fleshy, 
bifurcated, and capable of considerable protrusion, and working in a sheath. 
It is never venomous, as is commonly supposed. The male organs of 
generation are usually concealed within the cloaca; they are bifurcated 
and armed with recurved spines. The protrusion of this bifurcated and 
thickened penis, under certain circumstances, has no doubt given rise to 
the vulgar idea of the possession of distinct feet by the common snakes of 
the United States. The transverse slit of the cloacal orifice marks the line 
of distinction between the body and tail. One lung of the two is more 
generally abortive or rudimentary. 
Although destitute of limbs, the usual organs of motion, yet some 
serpents are capable of a very rapid progression. This progression may 
take place in various ways: thus the body may be straightened out 
entirely in contact with the ground, and a slow motion produced by the 
action of the scales and ribs, somewhat similar to that of the earthworm 
with its seta. Again, the body may be thrown into several undulations in 
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