THE TEETH 



263 



responsible for the term Labyrinthodontia which is applied 

 to the earliest amphibia. In snakes the teeth may be grooved 

 or even hollow and converted into poison-fangs. The 

 poisonous secretion passes in the groove or tube and is inserted 

 as with a hypodermic needle into the tissues of the prey. 



Living Chelonia have no teeth, but they were present in 

 the primitive fossil Triassochelys. The same applies to birds, 

 which are toothless to-day, but which originally possessed 

 teeth, as is shown by the fossil Archaeopteryx and others. 



The teeth of mammals and of those extinct reptiles which 

 were on the mammalian line of descent differ from those of 

 other vertebrates in that they are not all similar, but differ in 



Fig. 122. — The origin of teeth in the dogfish. 

 A, inner side of one half of the upper jaw, showing the rows of reserve 

 teeth ; B, section through the lower jaw ; the smallest teeth are the most 

 recently formed. Mc, Meckel's cartilage. 



shape in the various regions of the mouth. This condition is 

 called heterodont, as opposed to the homodont condition when 

 the teeth are all similar. 



The most anterior teeth are the incisors, and (except in 

 some Marsupials) they are never more than three in number 

 on each side in each jaw. In the upper jaw they are carried 

 on the premaxilla. Next come the canines, the premolars, 

 and the molars. The molars differ from the premolars in 

 that there is only one set of them, whereas the premolars are 

 represented by a lacteal or " milk " dentition followed by a 

 permanent set which replaces them. In a few mammals, such 

 as the toothed whales, the teeth are all similar, but this is a 

 secondary and degenerate condition. 



