356 



OPHIDIA. 



The skin is scaly, but the scales are purely epidermal struc- 

 tures placed on thickenings of the cutis ; osteoderms are absent. 

 The scales vary much in form, number and arrangement. When 

 they are small and overlap they are called scales, but when they 

 are large and only touch by their edges the term shield is applied 

 to them. 



a 



The scales of the head are dis- 

 tinguished according to their 

 position (Fig. 195). The mental 

 shields (q), i.e. the scales in the 

 mental groove on the ventral 

 surface between the rami of the 

 lower jaw, may be mentioned 

 as peculiar to snakes ; in front 

 of these two accessory labial 

 shields on either side form with 

 the median labial shield (o) the 

 anterior boundary of the mental 

 groove. The scales on the 

 ventral surface are for the most 

 part broad and invest the body 

 like transverse bands (Fig. 195 

 6, t), but small scales may also 

 be present here. The ventral 

 surface of the tail, on the other 

 hand, is as a rule, covered by a 

 double, or rarely by a single, 

 row of shields. 



Snakes moult several 

 times in the course of the 

 year. They strip off the 

 whole of the scaly epidermis. 

 The vertebrae are very 

 numerous and are divided 

 into caudal and precaudal 

 only. All the precaudal vertebrae, except the atlas, carry 

 ribs, and the caudal vertebrae have long transverse 

 processes. The vertebrae are procoelous, the posterior surfaces 

 being hemispherical. The zygapophyses are flat and look dor- 

 sally and ventrally and in addition to them the anterior side of 

 the neural arch carries above the neural canal a wedge-like 

 process — the zygosphene (Fig. 198) which fits into a correspond- 

 ing depression — the zygantrum — on the posterior surface of the 



Fig. 195. — Head ot Udlopeltis aesculapU. a 

 dorsal, b ventral view, c side view of head of 

 Tropidonotus viperinus (from Glaus, after 

 E. Schreiber). o frontal shield; b super- 

 ciliary shields ; c posterior snout, d anterior 

 snout, e parietal, / rostral shield ; g upper 

 labial shields ; h nasal shield ; i preorbital 

 shield ; it loreal, I postorbital, m temporal, 

 median labial shield ; p lower labial shields ; 

 q mental shields ; r jugular shields, s jugular 

 scales ; t ventral shields. 



