• Morphological Studies. 729 



And finally Prof. W. K. Parker (16, p. 398) in his beautiful mono- 

 graph on the skull of the Marsipobranch fishes mentions very briefly 

 the bright yellow horny teeth of the Myxinoids. 



So far as the Myxinoids are concerned, as will be seen in the 

 sequel, the negative results of all observers regarding the presence of 

 true teeth have arisen solely because no one has made microscopial sec- 

 tions of them. 



It is no easy matter to make decent sections through the head 

 of Myxine, and to obtain sections of Bdellostoma teeth one must 

 employ special methods, for the horn of those teeth will turn the edge 

 of any razor in existence. 



The horny teeth of Fetromyzon planeri. 



The teeth of this species are the simplest, and as F. E. Schulze 

 (18, p. 310) shewed they consist of simple horny plates resting on a 

 slight dermal papilla , and arising from a special groove at the base 

 of the papilla. I have figured such a tooth in longitudinal section in 

 fig. 11. The figure shews that the groove in which the tooth rests is 

 a horn-producing organ (hg). 



In some of the teeth (and this is represented in the particular 

 figure), one sees traces of a second horny plate below the first in the 

 deeper layers of the epidermis. This second plate appears to be a 

 reserve tooth. The only other point to be noticed is that the teeth of 

 P. planeri are very small. 



At no time in the life -history of Petromyzon planeri are real 

 teeth developed. I have studied their earliest development during 

 the metamorphosis or shortly after, and shall for the sake of com- 

 parison with the Myxinoids first describe it after I have given the ac- 

 count of their teeth. 



The horny teeth of Fetromyzon marinus. 



We have seen above that a reserve tooth may be sometimes 

 formed beneath the functional tooth of Petromyzon planeri: the pres- 

 ence of such reserve teeth becomes the rule in the marine form. In- 

 deed, one may almost describe the teeth of the latter as compound. 

 Each and every tooth of this form is made up of three horny cones, 

 one above the other like a nest of Chinese bones. This arrangement 

 is figured in diagrammatic section in fig. 12. 



Prof. Howes tells me he has known of this peculiarity of the 

 teeth of Petromyzon marinus for some time. The marine Petromyzon 



