H. F. Osborn — Mammalia in North America. 387 



order matters were reversed, for the first series persisted and 

 the second series were suppressed and persist as a rudimental 

 row of tooth caps buried in the jaw. 



Each dental series has an adaptive evolution of its own, in 

 Erinaceus the first series has an ancient and the second a 

 modern form ; in Ericulus both series are alike ; in the Bats 

 the first series is homodont the second is heterodont (Leche) ; 

 in the Edentates the first series is ancient and heterodont the 

 second is modern and homodont (Thomas, JRheinhardt) ; so 

 among the Cetacea and Ungulata. 



What deep and ancient clefts the different laws of succes- 

 sion mark between the Marsupials and these three Placental 

 groups. 



Primitive Heterodontism and Formida. 



Now that all mammals are led back to a distant diphyodont 

 stem, it is also true that the further we go back both in palin- 

 genesis and embryogenesis, the more widespread heterodont- 

 ism is — all modern homodontism proving to be secondary. 

 The simple conic teeth of the porpoise, for example, bear 

 a misleading resemblance to those of a reptile. Flower, 

 Weber, Julin, and Kukenthal agree that the ancestral whales 

 and edentates were heterodont and had a smaller number of 

 teeth than the existing forms. 



Heterodontism is then the second problem. When did the 

 division of the teeth into incisors, premolars, and molars occur, 

 before or after the Monotremes, Marsupials, and Placentals 

 separated? It is well settled that the canine was the first 

 maxillary tooth, and developed from the most anterior bi- 

 fanged premolar ; also, from the discovery of complete succes- 

 sion, we must now define the first molar as the most anterior 

 specialized or triconid tooth, not as the most anterior perma- 

 nent tooth. It seems to me we now find strong evidence that 

 the stem mammals had a uniform number of each kind of 

 teeth ; in other words, a uniform dental formula. The 

 Monotremes are most in doubt as the existing forms point 

 only to primitive heterodontism. It will be a great step for- 

 ward when we learn whether or not the Multituberculates are 

 Monotremes — the resemblance of their molars to those of the 

 Duckbill is very superficial, for the Duckbill upper molars 

 lack the intermediate row of tubercles universally seen in the 

 Multituberculates, and look to me rather like degenerate Tri- 

 tuberculate teeth. Cope has recently found in the Cretaceous 

 rocks a remarkable Trituberculate, which he names T/daeodon ; 

 the jaw of this animal is neither Placental nor Marsupial ; it 

 is like that of the Multituberculates — and both resemble re- 



