Morphological Studies. 739 



It might be supposed that the time at which they possessed true 

 tooth structure, i. e. that of the Myxinoid tooth, was so remote that 

 they had lost all trace of the mode of development of true teeth. 

 Or, again, we might consider these posterior teeth as secondary develop- 

 ments which never possessed the structure of Myxinoid teeth. The 

 latter explanation appears to be the true one. 



If the horny teeth of Fetromyson stood alone without the My- 

 xinoid teeth , which explain their development and morphology , we 

 could hardly assume that the Petromyzontidae ever had true teeth. 



The formation of these tooth sacs would be a very curious cir- 

 cumstance, and might make us cautious as to the morphology of the 

 Petromyzontoid teeth, but we should hardly be justified in assuming 

 the former existence of true dental structures in the group. 



The comparison of Myxinoid and Petromyzontoid teetli. 



The Myxinoid teeth bridge over the gulf between horny teeth 

 and true teeth, for they shew us the way in which starting from true 

 dental structures we may end in having only horny teeth. And not 

 only is this true for the Marsipobrauchs , it also holds for all 

 other cases of horny teeth. We see also the course along 

 which the teeth of some other Vertebrates: birds, reptiles and Mono- 

 tremes, have passed, in order to finally degenerate to purely horny 

 structures. In this sense horny teeth are substitution products of true 

 teeth. Nor is this all. I take it that the horny jaws of Chelonians 

 and birds were at one time substitutions of teeth. — The fusion of 

 the lingual teeth of the Myxinoids shews clearly how from separate 

 horny teeth there may ultimately be a compound horny plate form- 

 ed. It requires but the fusion of a number of such horny teeth in 

 the lower, and again in the upper jaw, to produce a biting apparatus 

 identical with that of a Chelonian. 



The horny carapace etc. of a Chelonian is a structure which is 

 endeavouring to strengthen and finally perhaps supplant and substi- 

 tute a series of dermal bones, and thus ultimately it too is a substi- 

 tution product of tooth scales. But here I am anticipating. In the 

 same way as the horny teeth of Petromyson are the degenerate remains 

 of true teeth, the scales of reptiles may fairly be considered as sub- 

 stitutions of the skin-scales of fishes. Leydig^) has shown that the 



1) In Scincus and Cyclodus among Lizards „the dermis beneath 



Zoolog. Jahrb. III. Abth. f. Morph. ^.y 



