COLUBRID WITH VERTICALLY MOVABLE MAXILLA. 85 



what enlarged, often in a vertical direction, showing a resemblance 

 to the state of things found in Causus. 



The first argument is easily disposed of, as in a number of 

 Opisthoglyphs the fangs are situated just below the prefrontal 

 (Miodon, Polemon, Brachyophis). Now in Xenodon the portion 

 of maxilla bearing the fang-like teeth will be found to be much 

 enlarged, and in a more or less vertical direction, and it only 

 remains for the last two teeth. to be furnished with grooves to 

 transform Xenodon into an Opisthoglyph with the fangs situated 

 below the prefrontal. Further, we have only to compare the 

 maxilla of Xenodon with that of the least specialized of the 

 Vipers, Causus, to see that merely a slight tilting up of the 

 maxilla of the former snake, with the loss of the few front 

 teeth and a very slight modification of the bone, is needed to 

 bring about a condition similar both in structure and mechanism 

 to that of Vipers. Thus Xenodon with its vertically movable 

 maxillae enables us to trace the probable evolution of this bone, 

 and the old view, recently revived, that Vipers are descended 

 from Proteroglyphs must, in my opinion, be abandoned. 



Mr. Hewitt in his papers states that the various experiments 

 on snake-venom seem to show that there is more in common 

 between the Proteroglyphs and the Vipers, than between the 

 Opisthoglyphs and either of the other divisions. That this is so 

 in the majority of cases has been demonstrated by Phisalix. It 

 should be borne in mind, however, that, as has recently been 

 shown by Fitzsimons, the poison of the most highly venomous 

 Opisthoglyph, Dispholidus typus, in its physiological action is 

 particularly characteristic of that of the South American Vipers 

 of the genus Lachesis. The physiological action of the venoms 

 can, therefore, have little importance in the settlement of the 

 broader problem of the classification of Snakes from the point of 

 view of descent. 



