THE TEETH OF MAMMALS 119 



or more distinct fangs. Implantation is the ex- 

 clusive mode of attachment among mammals, but 

 this is also imitated in the crocodiles and some 

 other reptiles. Heterodontism (having teeth of 

 different forms in the same jaw) is a mammalian 

 characteristic, but the lizards also have this fea- 

 ture to a limited extent, in which the tubercular 

 form begins to manifest itself. The number of 

 the teeth is reduced in mammals, though some, 

 as the dolphin, have a great number. Succes- 

 sion is reduced, as there are never more than 

 two sets in the mammals and there may be many 

 in the reptiles. 1 The elaboration of the crowns 

 by the duplication of the primitive cone is an ex- 

 clusively mammalian character, for the repetition 

 must be shown in the root also. The primitive 

 cone is of course reptilian, and from this all 

 teeth, even the most highly specialized, were de- 

 veloped. The series is continuous throughout the 

 vertebrate series, and the homologies are ap- 

 parent in all stages. 



Descriptive. The lowest order of the Mam- 

 malia is that of the Monotremata, which includes 

 but two species, the Ornithorynchus (or Duck- 

 bill mole) (Fig. 38) and the Echidna (ant-eater). 

 These are, dentally, the lowest forms of living 

 mammals, and present the anomaly — like the low- 

 est species of fishes and reptiles — of possessing 



1 Some claim that each molar of the elephant represents a set of teeth, 

 in which case there would be six sets. — Editor, 



