136 K. BKOOM. 



ill that by showing how the degeneration has taken place we 

 get a clue as to the mode of original development. 



In tracing the phylogeny of mammalian molars it is to be 

 regretted that we have so few specimens of upper molars of 

 the Jurassic and Cretaceous types^ but we have sufficient 

 lower molars to guide us some way on sure ground. We can 

 safely conclude that a series of simple conical teeth in some 

 reptilian ancestor gave rise to more or less simple teeth in 

 some early mammalian type. Even without leaving the 

 reptilian groups we find teeth becoming complicated by the 

 addition of cusps. In the Pareiasauria the teeth have a series 

 of lateral cusps which give them an appearance not unlike 

 those of Zeuglodon. In the Dinocephalia we find an elaborate 

 development of cusps in the incisors of Titanosuchus and 

 Delphinognathus. Among the Anomodontia well-deve- 

 loped cusps are formed on the molars on Endothiodon, 

 Opisthoctenodon, and Pristerodon. When we reach 

 the Cynodontia we find cusps on the molar teeth of most of 

 the genera. These reptilian cusps have propably no direct 

 connection with those formed on the mammalian molars, but 

 are interesting as showing that the mammal-like reptiles have 

 the same potentiality for cusp development. 



The probability seems to be that the earliest mammals had 

 simple conical teeth such as certainly occur in some Cyno- 

 donts, e. g. Bauria, and that gradually cusps developed on 

 the sides to lengthen them out and make them better cutting 

 instruments. Most of the early mammalian jaAvs have teeth 

 formed of a main cusp with a small anterior and posterior 

 cusp, and we are probably safe in concluding that all later 

 mammalian molars are derived from this Protodont type or 

 Triconodont type. 



Those who hold that the triconodont type gave rise to a 

 Tritubercular type where the three cusps form triangular 

 crowns may be perfectly correct, but the evidence is less con- 

 clusive. Teeth, such as seen in Spalacotherium and 

 Menacodon, seem to show that in the lower molars at least 

 there is a tendency to form triangular crowns. In the Sea, 



