164 



OPHIDIA. 



Other snakes, as the Pythonidoe and Erycidoe, also have rudiments of hind limbs ; but in 

 other respects, especially in the structure of the mouth, they have the Ophidian character 

 so much developed that there cannot be any doubt as to their position in the system. 



The organs of locomotion for the exceedingly elongate body of the snakes are the ribs, 

 the number of which is very great, nearly corresponding to that of the vertebrae of the 

 trunk. Although their motions are in general very quick, and may be adapted to every 

 variation of ground over which they move, yet all the varieties of their locomotion are 

 founded on the following simple process. When a part of their body has found some 

 projection of the ground which affords it a point of support, the ribs, alternately of one and 

 the other side, are drawn more closely together, thereby producing alternate bends of the 

 body on the corresponding side. The hinder portion of the body being drawn after, some 

 part of it finds another support on the rough ground or a projection ; and the anterior bends 

 being stretched in a straight Ime, the front part of the body is propelled in consequence. 

 During this peculiar kind of locomotion the numerous broad shields of the belly are of great 

 advantage, as, by means of the free edges of those shields, they are enabled to catch the 

 smallest projections on the ground which may be used as points of support. A pair of ribs 

 correspond to each of these ventral shields. The snakes are not able to move over a perfectly 

 smooth surface. 



The snakes are naked, like all other reptiles : that is, no separate epidermal productions 

 form the external integument, but the epidermis is very regularly laid into scale-like folds 

 on the back and the sides, and into broad imbricate transverse shields or plates on the belly. 

 The epidermis on the head is generally divided into non-imbricate shields. The form of the 

 scales and ventral shields, the number of their longitudinal and transverse series, and the 

 shape and arrangement of the head-shields are of the greatest value for the distinction 

 of the species and genera. The followdng woodcuts of the head of a snake with the 

 normal arrangement of the shields {Ptyas korros) will explain the terms used for their 

 denomination : — 



i P a 



r. Rostral. 

 /'. Anterior frontal. 

 /, Posterior frontal. 

 V. Vertical. 

 s. Supraciliary, 

 0. Occipital. 



n, n . Nasals. 

 I. Loreal. 

 a. Anterior ocular or orbital ; 



anteocular or praeorbital. 

 p. Posterior ocular or orbital ; 

 postocular or postorbital. 



u, u. Upper labials. 

 t, t. Temporals. 

 m. Median lower labial or 



mental. 

 **. Lower labials, 

 c, c. Chin-shields. 



