386 H. F. Osborn — Mammalia in JVorth A 



merica. 



the second series is represented by the " permanent incisors 

 canines, and premolars " and rudiments of dental caps beneath 

 the true molars. The mammals early began to diverge from 

 this primitive diphyodontism in many ways ; apparently adapt- 

 ing the first and second series, respectively, to their infant ard 

 mature feeding habits ; losing parts or all of one series or the 

 other, and in some cases pushing teeth of the second series in 

 among the first ; this intercalation has been a most confusing 

 factor to us. 



In the Marsupials (Kukenthal) almost the entire first series 

 became permanent ; thus from the Jurassic period to the 

 present time only a solitary fourth premolar of the second 

 series has pushed out its elder sister tooth, and Rose has 

 observed that' an outer upper-incisor also pushes up from the 

 second series ; the remainder of the second series still persist 

 as rudimental dental caps beneath the first, even beneath the 

 first and second molars ! There are wide variations among the 

 Placentals ; thus in the lowest existing forms, the Insectivora, 

 Leche finds that in the Shrew (Sorex) the second series is sup- 

 pressed entirely, while in the Hedgehog {Erinaceus) of the 

 twelve permanent teeth in the anterior part of the jaws five 

 belong to the first series and seven to the second. We thus 

 meet with the paradox, that among the " primitive " Marsupials 

 and Insectivores the regular reptilian succession was early 

 interrupted, while in all the " higher " mammals the reptilian 

 succession of two series was retained in the anterior part of 

 the jaw. Beneath the posterior highly specialized molar teeth 

 of both Marsupials and Placentals, the second teeth were early 

 suppressed, although in the Edentates, which also originally 

 had specialized molars, there is a typical succession of seven 

 teeth behind the canine. These discoveries prove that the 

 whale teeth, like their paddles, have acquired a secondary 

 adaptive resemblance to those of the Icthyosaurs. How did 

 the single and simple teeth of the Edentates and Cetaceans 

 develop ? Clearly by retrogression. As Leche points out in 

 the aquatic Carnivora, in which the first series are degenerat- 

 ing, the single-series condition (monophyodontism) advances 

 step by step with retrogressive simplification of the tooth form 

 (homodontism) ; thus in the true seals, the eared seals and the 

 walruses, as the permanent teeth become simpler, the milk 

 teeth become smaller. The Edentates, so widely separated 

 genetically, parallel the seals in tending to suppress the first 

 series of teeth and simplify the crowns of the second series at 

 the same time. We might jump to the conclusion that this 

 gives us an explanation of the homodont and apparently mono- 

 phyodont condition of the toothed whales, especially as it has 

 been supposed they sprang from aquatic Carnivora, but in this 



