OPHIDIA. 



407 



may be distinguished from the Viperine snakes, is in the shape 

 and armature of the head. Tn the Viperina, as before said, the 

 head (figs. 157, 158) is triangular, broadest behind, and sepa- 

 rated from the neck by a more or less marked diminution in 

 the diameter of this latter part. The scales, too, which cover 

 the head are of small size. In the Colubrine snakes, on the 



Fig. 158. A, Head of Colubrine ^daz (Coluber natrix); B, Head ofViperine Snake 

 (Pelins berus); C, Head of Blind-worm (A nguis fragilis), one of the serpentiform 

 Lizards. (After Bell.) 



other hand, the head is not markedly triangular, and gradually 

 tapers off into the neck, whilst the upper surface of the head is 

 usually covered with large shield-like plates or " scuta " (fig. 

 158, A). 



DISTRIBUTION OF OPHIDIA IN TIME. The Ophidia are not 

 known to occur in any Palaeozoic or Mesozoic deposit. The 

 earliest known traces of any serpent are in the Lower Kainozoic 

 Rocks, the oldest being the Palceophis toliapicus of the London 

 Clay of Sheppey. The nearly-allied Palczophis typhceus of the 

 Eocene beds of Bracklesham appears to have been a Boa-con- 

 strictor-like snake of about twenty feet in length. In some of 

 the later deposits have been found the poison-fangs of a veno- 

 mous snake. Upon the whole, however, the snakes must be 

 looked upon as a comparatively modern group, and not as one 

 of any great geological antiquity. 



