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ordinary tooth ^). It is composed of very hard, large, corneal cells 

 possessing a large rounded nucleus, and presenting a striated appear- 

 ance. On the outer side these striae are especially well marked, and 

 these cells appear to me to be directly comparable to the odontoblasts 

 of a developing tooth. No enamel larger is formed, and there is no 

 specially modified row of epithelium cells having the appearance of 

 the socalled „enamel organ" of other teeth. The tooth seems to have 

 lost its power of forming a layer of enamel, or one may rather say, 

 the enamel layer is replaced functionally by the horn. The centre 

 of the dentine cone is a true pulp cavity with blood vessels etc. etc. 



It will be seen from thes brief description that the tooth of the 

 Myxinoid diöers in some particulars from the teeth of other Vertebrates. 

 The first difference consists in the presence of a horny layer, the 

 second is one in which it only differs from some Vertebrates, the 

 absence of an enamel layer. A layer of enamel does not occur in 

 all cases, though according to C. S. Tomes an „enamel organ" always 

 occurs in developing teeth. As no one has yet seen a developing 

 tooth in Myxine it is of course impossible to say if the animal pos- 

 sesses such on „enamel organ". One cannot say the adult does, at 

 any rate not according to the current ideas of that structure. 



Whether the socalled enamel organ plays any part in the forma- 

 tion of the enamel, or whether the whole tooth is formed by the odon- 

 toblasts and is possibly entirely an endodermic structure, i. e. a body 

 formed entirely in the dermis as Huxley believed and as Prof. Howes 

 suggests to me, are points I will not here attempt to discuss. 



I intend to give a fuller account with figures of these remarkable 

 teeth, and will now merely content myself with drawing attention to 

 the important bearings of the discovery on the morphological position 

 of the Cyclostomata. 



Freiburg i. Br., 31. Dez. 1887. 



1) Since writing the above I have been able to detect in a fortu- 

 nate section of the tooth of Bdellostoma a small structureless cap of 

 either dentine or enamel at the apex of the odontoblast cone. In My- 

 xine, also, on careful search very minute traces of such a deposit are to 

 be found. 



