H. F. Oshorn — Mammalia in North America. 385 



The brilliant discoveries of Kukenthal, Leche, and Rose 

 begin to show how in various ways the mammals early mod- 

 ified the regular succession of all the teeth by suppression of 

 parts of the multiple series ; this is the first thing to consider. 

 The next is how heterodoutism arose, how the conic rows of 

 teeth were specialized in different parts of the jaw for three 

 or four functions ; as a certain number of teeth took up each 

 function, the question arises whether this number or dental 

 formula was ever the same in all the mammals, for we know 

 it is very different now. After the teeth were thus divided, 

 some functions became more important than others, and estab- 

 lished a monopoly, causing first a marked difference in the 

 relative development of the series, which we may express in a 

 dental curve, resulting finally in a loss of certain teeth. In 

 the meantime began the special evolution of the form of the 

 back teeth, or molars. Was this alike in all mammals, was it 

 tritubercular ? It is surprising how many problems of early 

 relationship are at stake in these simple processes. 



Primitive Diphyodontism. 



What does succession really consist in ( It now appears 

 that Baume was right in denying that the first tooth is the 

 mother of the second ; for the teeth of the lower as well as 

 the upper series, spring from the common epithelial dental 

 fold (Schmelzleiste) which dips down from the surface and 

 extends the whole length of the jaw ; at intervals it buds off 

 the dental caps (Sehmelzkeim) of the first series: after these 

 are separated off, the dental fold sinks and buds off the dental caps 

 of the second series, always below and inside the first ; thus the 

 fold is the mother and the caps are sisters, twins, or triplets, 

 according to the number of the series. In all young mam- 

 mals, including the traditional monophyodont Cetaceans and 

 Edentates, and excepting only the still unexplored Monotreme 

 embryos, traces of two series of teeth have been found. Both 

 Leche and Rose have detected evidence that the dental fold 

 sometimes buds off parts of a third series, thus explaining the 

 occasional reversion of supernumerary teeth on the inner side 

 of the second series, and Leche has seen traces of budding 

 preceding the first series — thus giving us vestiges of four suc- 

 cessions ! 



All our perplexities as to the relations of the milk and per- 

 manent teeth, and the ingenious but mistaken hypotheses of 

 Baume, Flower, Wortman, have sprung from our want of 

 evidence of the regular and complete diphyodontism of the 

 stem mammals. The solution in brief is that the " milk teeth " 

 and the " true molars " are descended from the first series, while 



