728 Dr, J. BEARD, 



wie wir dies bei Trionyx gesehen haben , in embryonaler Zeit noch 

 echte Zähne anlegen — im Sinne einer regressiven Metamor- 

 phose aufzufassen." 



For all the groups Wiedersheim mentions, except one, most 

 zoologists would find little difficulty in accepting this dictum. But 

 the true gnathostomatous nature of the Marsipobranchs has been dis- 

 puted by men of such high scientific rank as Profs. Balfour, Haeckel 

 and Gegenbaur; and the horny teeth are regarded by them as for- 

 mations with no relationships to true teeth. The facts to be here re- 

 corded modify their conclusions, and lend support to the views so long 

 upheld by Profs. Dohrn and Huxley (4, p. 34). 



At about the time I discovered the true teeth of Myxine, Poul- 

 TON (17) had found teeth in young Ornithorhynchus. It is known 

 that the horny teeth of Amphibian larvae are secondary developments. 

 Acipenser possesses teeth in the larval stages (13). The whalebone 

 whales also are furnished with teeth in the foetal condition, and the 

 earliest birds are also known to have had teeth. 



So now one may confidently assume that teeth are characteristic 

 of all Vertebrates above the Marsipobranchs. Nor need the assertion 

 stop here, but must be made to include the Myxinoids, and most prob- 

 ably the more degenerated Petromyzontidae. 



Historical. 



This section need not detain us long, for very little is known 

 about the teeth of the group. 



To go no further back than J. Müller (15, p. 84) ; in his beautiful 

 researches one finds the statement that the teeth of the Cyclosto- 

 mata are horny, and beyond this only the topography of the teeth in 

 the mouth is mentioned. 



F. E. Schulze (18, p. 310, Tf. 17, Fig. 10) examined the 

 teeth of Petromyzon fluviatilis, and described correctly the simple 

 horny plates, showing that they rest on a slight dermal papilla, and 

 fit into special epidermal depressions at the base of the papilla. 



Langerhans (14, p. 39) in a very valuable paper on the ana- 

 tomy of the freshwater Petromyzon could naturally only confirm 

 Schulze's results regarding the teeth. 



C. S. Tomes (21 , p. 153) speaks of the horny teeth of the Cy- 

 clostomata, and points out how entirely different they are to the 

 teeth of the rest of the Vertebrata. 



