582 OPHIDIA CHAP. 



pidae and Glauconiidae only. The premaxilla is unpaired and 

 small, and is rarely furnished with teeth. The latter are always 

 sharp and recurved, and are lodged in sockets upon the edge of the 

 supporting bone, with which they become firmly ankylosed. 

 There is a perpetual succession of teeth. In the majority of 

 Snakes teeth are carried by the maxillaries, palatines, pterygoids, 

 and dentaries, rarely by the premaxillaries. The palatal teeth are 

 restricted to the palatines in Oligodon, Dasypeltis, and Atractaspis 

 only. 



Peculiar modifications prevail in the poisonous Snakes. 

 Those maxillary teeth which are at their base in connexion 

 with the openings of poison - glands (modified upper labial 

 glands), either have a furrow on the anterior side (Proteroglypha 

 if the anterior teeth are grooved, e.g. the Cobras ; Opistho- 

 glypha if some of the posterior teeth are grooved), or the groove 

 is converted into a canal, as in the Solenoglypha or Viperidae. 

 The special modification of the maxillaries of the vipers with 

 their long poison-fangs is described on pp. 587 and 637. 



The orbit is generally closed behind by the postfrontal. 

 Quadrato-jugal, postfronto- squamosal, and other arches are 

 absent, so that the temporal fossa is quite open (see Fig. 156, p. 

 597, and Fig. 155, p. 596). The occipital condyle is distinctly 

 triple. The mandibles are composed of several bones, but the 

 coronoid is absent in the Xenopeltidae, Colubridae, Ambly- 

 cephalidae, and Viperidae ; it is large in the Boidae, reduced to 

 a nodule in the Ilysiidae. 



The parietals are always fused into a large unpaired bone, 

 which generally forms a sharp crest and partly overlaps the 

 occipitals ; there is no interparietal or pineal foramen. 



The vertebral column consists of many, often nearly three 

 hundred vertebrae, and these skeletal segments correspond in 

 number with those of the ventral and transverse scales of the skin. 

 The vertebrae are procoelous ; in addition to the anterior and 

 posterior zygapophyses they have a pair of accessory articulations 

 on the neural arches, dorsally to the zygapophyses ; — the " zygan- 

 trum " carried by the posterior end of the neural arches, its 

 articular surfaces looking upwards ; and the " zygosphene " carried 

 by the anterior end and looking downwards. Such accessory 

 articulations occur also in a few Lizards, e.g. Iguanidae. The 

 vertebrae of many Snakes have unpaired vertical, blade -like 



