148 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the tip which, as it were, guides and saves the 

 poisonous fluid. 



As a further aid to the more extreme venomous 

 forms, which can only subsist by this mode of attack- 

 ing their prey, and which are eminently liable to have 

 their organ of offence broken in the act of "striking," 

 the reserve teeth are arranged in a manner which 

 seems to be unique in the animal kingdom. Instead 

 of a single series of reserve teeth set in one and the 

 same line with the tooth in active function, there are 

 two rows, in each of which the pair of teeth are 

 almost of the same age and grade of development. 

 When, therefore, the active tooth is lost, that in the 

 other line, which is lying beside it (Fig. 65), is ready 

 at once to move forwards into a little different 

 position, and to take on its function ; by this means 

 the fang is replaced with a minimum loss of time (C. 

 S. Tomes). 



When we come to the Mammalia, where, as 

 has been already said, teeth are never found except in 

 the mandibles, maxillae, and premaxillae, we are met, 

 at the outset, with an arrangement of which, at 

 present, it seems impossible to afford any altogether 

 satisfactory explanation. There are never more than 

 two sets of teeth, one of which is temporary or 

 milk, and the other permanent* the teeth of 

 these two sets differ in form, size, and number. 

 In some cases the milk teeth are never of any use ; 

 such mammals may be conveniently spoken of as 

 monophyodont, while those in which there are two 

 sets may be similarly called diphyodont. At the 

 same time it must be carefully borne in mind that 

 that there is no sharp delimitation between these two 

 groups. In marsupials and guinea-pigs there is only 

 one milk molar ; in the rabbit the milk incisors dis- 

 appear before birth, and among edentates only one 

 species (Tatusia peba) is known to have milk-teeth. 



