171 



merely one plate of horn but three plates one above the other are 

 developed M- And even by macroscopical examination one readily distin- 

 guishes three cones of horn one above the other, like a nest of three 

 Chinese boxes. As each of these layers arises from a separate de- 

 pression of epidermis, and as these depressions are not temporary or 

 evanescent organs, but seem continually to form horn ; I cannot speak 

 of the two additional inner horny layers as reserve teeth, it appears 

 to me more likely that they are there for the purpose of strength- 

 ening the tooth, not of furnishing new teeth, for as each tooth is 

 worn way at the apex fresh horny matter is formed below and pushed 

 forwards. 



So far as the teeth of the Petromyzontidae go, no points of resem- 

 blance with those of the higher Vertebrates are to be met with. 



I have recently ^) endeavoured to point out the morphology of 

 an interesting structure in the Myxinoids and now I describe my finds 

 in the minute anatomy of their teeth. Myxine and Bdellostoma •'' ) 

 have both been examined; they present essentially the same cha- 

 racters, the sole difference is that the teeth of Bdellostoma are 

 stronger and larger than those of Myxine. 



I will here briefly describe the appearance in vertical frontal 

 section of the large median tooth of Myxine, merely remarking that 

 the account also holds for the lingual teeth. 



The outer layer is composed of horny matter formed in exactly 

 the same way as that of P. planeri, i. e. from epidermal depressions 

 at the base of the tooth. It is well developed in both Myxine and 

 Bdellostoma. 



Within the horny layer is another of stratified epithelium com- 

 posed of moderately flattened, spindle-shaped cells. The deepest layer 

 of this is slightly more regular and cubical, and rests on the main 

 body of the tooth. This is a thick hard conical cellular structure. 

 I believe it is calcified but as yet I have not examined chemically 

 the basis of its hardness. This cone has all the appearance 

 and structure of the odontoblast portion of an 



1) In P. planeri traces of a deeper layer of horn are met with in 

 some of the teeth. ^ 



2) Beard, The old mouth and the new. Anat. Anzeiger 1888, No. I, 

 p. 22. 



3) I have to thank Prof. Howes for the kind gift of the specimens 

 of Bdellostoma and Petromyzon marinus on which my work was made. 



