630 



REPTILIA. 



structure. For the external shape is in great part an adapta- 

 tion to the mode of life, to the habit of creeping through 

 crevices or among obstacles. But the limblessness of 

 serpents is not a merely superficial abortion ; there is no 

 pectoral girdle nor sternum, and never more than a hint of 

 a pelvis. 



GENERAL CHARACTERS. The skin is covered with scales \ 

 and the outermost epidermal layer is periodically shed in 

 a continuous slough. 



There are never any hints of anterior appendages, girdles, 







V 1 



FlG. 344. Anterior view of 

 Python's vertebra. 



neural spine; ZS., zygo- 

 sphene (a projecting wedge); PR.Z., 

 pre-zygapophysis (smooth articular 

 surface seen from above); ./?., 

 articulation-surface for a rib ; HY. % 

 hypapophysis. 



FIG. 345. Posterior view of 

 Python's vertebra. 



ZA. t zygantrum, a double cavity 

 for the zygosphene; PT.Z., post- 

 zygapophysis (smooth articular 

 surface seen from below); T.P., 

 transverse process ; C., centrum. 



sternum^ or episternum ; but in pythons, boas, and a few 

 others, there are rudiments of a pelvis, and even small clawed 

 structures which represent hind-legs. 



The mouth is expansible ; maxillce, palatines, pterygoids, 

 and quadrates are movable ; and the rami of the mandible 

 are connected only by elastic ligament The teeth are fused to 

 the jaws ; there are no movable eyelids. Snakes have no 

 external ear openings nor drum, nor tympanic cavity, nor 

 Eustachian tube. The nostrils lie near the tip of the 

 head. 



