AND TEETH OF THE OPISTH06LYPHA. 



319 



of the jaw (text-fig. 20) which disables the prey before it is 

 carried further into the mouth to the region of the poison fang. 

 In order to do away with unnecessary apparatus and provide one 

 fang which would serve all purposes, the poison fang has shifted 

 forward right to the front of the mouth, dragging with it the 

 duct of the poison gland. This gave rise to vipers, where we know 

 that the poison apparatus is perfect in all ways and that the 

 other teeth have disappeared. Probably evolution has taken 

 place in this way, as already pointed out by Boulenger : 

 "the series (in Opisthoglypha) culminating in such forms as 

 have the maxillary bone much abbreviated, the solid teeth 

 reduced to two or three, and the fang extremely large and deeply 

 grooved. If we turn to the skull of the least specialised among 

 the Viperidae, we see that the poison fangs are situated on the 

 posterior extremity of the maxillary bone, close to its articulation 

 with the ectopterygoid, a condition which is identical with that 

 of Opisthoglyphous Colubrids. It is therefore clear to me that 

 the ^Hperids have been deri\fed from the Opisthoglyphs." 



Summary. 



Text-figure 22. 



s.l.g. 

 Lateral view of head. 



1. Hypothetical ancestor. 



j7.y Hooth 



Transverse section '^^fjlfy^':^^ 



Gland in the region of maxilla undifferentiated : teeth all equal and peg-like. 

 All the ducts open in the buccal cavity far away from teeth. 



Text-figure 23. 



duct s./.y. ^ 



2. Dendrophis pictus. 



A small portion, near the posterior end, of the gland {p.g.) differentiated. The 

 duct of this portion moved inward to open near the hinder tooth (_/). 

 The latter has become different from others by being more bent inwards, 

 and also has developed a slight cutting-edge {c.e.t.) on the inner side. 



