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 vertebrae 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 eo-crs are generally protected by a calcareous 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. 



Order 2. Ophidia. 



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 beino- 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 w>.h 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 vertebrae 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 setae. Again, the body may be thrown into several undulations in 

 466 



