CHAPTER III. 



PHYLOGENY OF VENOMOUS SNAKES. 



The distinction between the non-poisonous and poisonous snakes is the 

 presence in the venomous species of a special poison apparatus, consisting 

 of a poison gland and poison fangs — teeth especially adapted for injecting 

 the venom into the tissue of the victim. The poison gland is in communica- 

 tion with the poison fang through an excretory duct which originates in the 

 former. The poison fang is distinguished from ordinary maxillary teeth by 

 the presence of a longitudinal groove or a canal running along the axis of the 

 fang on the frontal side. The canal ends in a slit-like orifice on the frontal 

 surface near the apex. A mere presence of the poison gland alone does not, 

 however, entitle a snake to the rank of Serpentes venenata, but it requires 

 the simultaneous presence of the poison fang and the poison gland. The 

 composite and complex nature of the poisonous apparatus creates conditions 

 which directly or indirectly influence the degree of danger from a bite of a 

 poisonous snake. The efficiency of a venomous bite depends to a consider- 

 able extent upon the size and position of the poison fang, the nature and 

 amount of the venom, and also the habit of the snake. These factors decide 

 the degree of danger of the poisonous snakes and render it convenient to group 

 them into dangerous and non-dangerous species. 



The evidence that the poison apparatus is the result of progressive evolu- 

 tion is made clear in the morphology of the maxillary teeth alone. In the 

 innocuous species the maxillary teeth are small, uniform, and solid, and are 

 designated Aglyphodont. The next group of species has one or more grooved 

 teeth in the rear of the maxilla, but no grooves in the anterior teeth. This 

 group has received the name of Opisthoglyph, and must be considered as the 

 primitive form of the venomous snake. The next step of advancement is 

 shown in certain species in which the anterior maxillary tooth or teeth are 

 grooved in varying degree. The groove may be so deep that the edges pro- 

 trude forward to fuse together and inclose a complete canal. In such case 

 the junction line of the edges is easily recognizable. Behind the grooved or 

 canaliculate teeth there may be some small solid teeth in some species. This 

 group is known under the name of Proteroglyph. Here the grooved teeth 

 attain a larger size than the rest of the maxillary teeth. The last and highest 

 stage of evolution of the poison fang is, however, seen in the group called 

 Solenoglyph, in which all maxillary teeth embrace a longitudinal tubular 

 duct. Only in the newborn specimens a faint fusion mark along the 

 frontal median line of the fang is visible, while no trace persists after ma- 

 turity. The size of the fang reaches large proportions, and the fang is erectile 

 through the movable joint of the maxillary bone with the prefrontal bones. 



A systematic inquiry into the relation between the poison fang and poison 

 gland in their evolutional chronology is not without certain bearing on the 

 question of the origin of poisonous snakes, and on the evolutional relation of 



