DENTITION 55 



gum at the inner side, and becoming fixed to the bone 

 soon after a vacancy occurs. Such replacement teeth, 

 of different grades of development, form several series, 

 so that in a snake like our common Tropidonotus the 

 mouth may contain four times as many teeth as are 

 functional, without reckoning different earlier stages 

 of tooth germs which escape ordinary observation, 

 being placed vertically one above the other. 



Three types of teeth, connected by every inter- 

 mediate step, are distinguished : the solid, the 

 grooved, and the canaliculated or tubular, so-called 

 " perforated " ; the third, as we shall explain, being 

 only a further modification of the second. In the 

 grooved tooth, a sulcus runs along the anterior or 

 outer surface, its object being to convey into the 

 wound the secretion of a poison gland. It varies in 

 depth according to the species, and may be so slight 

 as to escape detection without a very strong magnify- 

 ing glass. In some the sulcus may be very deep and 

 wide, forming a canal round which the tooth folds to 

 the extent of its borders nearly meeting ; from this 

 condition the so-called "perforated" fang is derived 

 through the complete fusion of the borders of the 

 tooth, and the obliteration of the line of union except 

 at each extremity. The structure of such a fang may 

 be best understood by imagining a tooth, lined all 

 round with the same layer of dentine and enamel, 

 being flattened out in a vertical plane and then 

 folded over, the outer edges coalescing on the front 



