ON A COLUBRID WITH VERTICALLY MOVABLE MAXILLA. 83 



7. On a Colubrid Snake {Xenodon) with a vertically movable 

 Maxillary Bone. By E. G. Boulenger, F.Z.S., Curator 

 of Reptiles. 



[Received November 10, 1914 : Read February 9, 1915.] 



(Text-figure 1.) 



Index. 



Page 

 Structure or Morphology. Maxillary bone in Snakes ... 83 

 Physiology. Importance of physiological action of the 



venoms of Snakes on classification 83 



iEtiology. Evolution of the maxillary bones in Snakes... 85 



In Vipers the maxillary bones, to which the poison-fangs are 

 firmly attached, are movably articulated to the prefrontals and 

 ectopterygoids, the poison-fangs being, when at rest, folded 

 against the roof of the mouth and becoming erected, or even 

 thrust forward, when the animal is about to strike. This ver- 

 tical mobility of the maxillary bone, which gives these snakes 

 such a mechanical advantage when they are about to strike, has 

 always been regarded as essentially characteristic of the members 

 of the family Viperida?. The Society recently received from 

 Mr. W. A. Smitbers, C.M.Z.S.,a generous donor to its collection, 

 a specimen of Xenodon merremi, an aglyphodont colubrid in- 

 habiting Brazil and Paraguay, which is characterized by an 

 extremely short maxillary with only six or seven teeth, followed 

 after an interspace by a pair of strongly enlarged but likewise 

 solid, ungrooved fangs. On taking the snake from the box in 

 which it was packed and catching hold of it behind the head, 

 I was most surprised to see the creature, on opening its mouth 

 in an attempt to bite, erect and depress its fangs in a thoroughly 

 Viperine manner. Further observations showed that the mobility 

 of its maxilla was so great that the fangs could be not merely 

 erected, but thrust forward and sideways, revealing the fact that 

 the mechanism in this snake is more perfect than in a large 

 number of Vipers of similar size. 



This discovery of a solid toothed Colubrid with a vertically 

 movable maxilla is of special interest, as I think it goes a long 

 way towards settling the problem, so often discussed, of the 

 derivation of the Viperine maxillary bone. The Viperida? were 

 formerly believed to have sprung from the Proteroglyph Colu- 

 bricls. In the Catalogue of the British Museum, published in 

 1893, my father, G. A. Boulenger, F.R.S., expressed the opinion 

 that the poison apparatus of the Vipers was in all probability 

 derived from the Opisthoglyphs. Later, in a paper published in 

 the Proceedings of this Society, he pointed out that, from the 

 Aglyphodont forms in which the teeth increase in size posteriorly, 

 we are gradually led to the Opisthoglyphs, which can be differ- 

 entiated only by the presence of more or less deep grooves on the 



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