360 



OPHIDIA. 



canal open at each end. The poison gland opens at the base of the fang 

 and its secretion passes into the canal. When the mouth is shut, the 

 fangs are directed backwards and lie along the roof of the mouth. \^hen 

 the mouth opens (Fig. 198) the ventral end of the quadrate, which when 

 at rest is inclined backwards, is brought forward. The pterygoid is thus 

 thrust forward and the pterygo-palatine joint bent. This forward move- 

 ment of the pterygoid is conveyed by the transpalatine bone to the maxilla 

 and causes it to rotate through about 90° upon its prefrontal articvilation, 

 in such a way that the surface to which the fang is attached is carried 

 forwards and ventralwards, and the fang is erected, i.e. is made to project 

 downwards at the front end of the mouth (Fig. 198). 



There is no trace of anterior limbs or of shoulder girdle in any 



snake, and in most 

 PrJ j 'i-' P Ocs 



W \ ^ 1 ^^ 



da 



the pelvis and 

 hind limbs are also 

 absent. But traces 

 of the pelvis and 

 sometimes even of 

 the hind limbs are 

 present in a few 

 forms {Typhlopidae, 

 Boidae, Xenopeltis). 

 The vestige of the 

 pelvis seems to be 

 restricted to a single 

 bone on each side, 

 longitudinally dis- 

 jDOSed and embedded 

 in the muscle near 

 the anus ; it is 

 probably the ischium. To these, very short vestiges of hind 

 limbs carrying claws are frequently attached. 



Alimentary Canal. The teeth are acrodont and ankylosed 

 to the jaws. They are sharp-pointed and curved backwards. 

 They are usually absent from the iDremaxillae, but are present 

 on the maxillae, palatines, and pterygoids (occasionally absent 

 from the pterygoids) and on the dentaries. They chiefly serve 

 to hold the prey while it is being swallowed. In most of the 

 non-poisonous snakes the teeth are conical, but in the others 

 and in all the poisonous forms some of the maxillary teeth are 

 grooved in front. This groove is in the most poisonous forms 



Fig 198.- 

 aiticular 

 frontal 



—Skull of Crotnlus horridus (from Claus). Art 

 ; Bs basisplienoid ; D dentary ; ht ethmoid ; F 

 Jmx premaxilla ; Mx maxillary ; N nasal ; Ocb 

 basioccipital ; Ocl exoceipital ; Ocs supraoccipital ; P 

 parietal ; P/ postfrontal ; Pr prootic ; PI palatine ; Prf 

 prefrontal ; Pt pterygoid ; Qu quadrate ; Sq squamosal ; 

 Tr transpalatine. 



