74â ^^'- -J- BEARD, 



lampreys are descended, ultimately at least, from forms provided 

 with placoid scales". 



And now Dr. Scott finds it „necessary to qualify this opinion, he 

 thinks that, such a conclusion, however, does not by any means commit 

 us to the view that the Myxinoids are degenerate descendents of some 

 Gnathostomatous group, as this is no more implied in the possession 

 of ordinary calcareous teeth than in the presence of the horny teeth, 

 which the group has long been known to possess". 



This argument, which has been made to me by others, is in simple 

 words, that true teeth do not postulate the existence, now or in the 

 past, of true jaws. The argument is only used for the Marsipo- 

 brauchii, in which the morphology of head is and has been difficult 

 to solve. If the structures found in the head of a Myxinoid cannot 

 be homologised with structures in the head region of an ordinary 

 Vertebrate or Vertebrates then the argument may be worth some- 

 thing. 



But, considering the comparisons and discoveries in the morpho- 

 logy and development of the Marsipobranchs which have been made 

 by J. Müller, Huxley, Parker, Dohrn, Ahlborn and by Scott 

 himself it is not too much to say that the presence of true teeth in 

 the Myxinoids does postulate the former existence of true jaws in 

 the whole group. 



Comparison of results and general considerations. 



It is generally agreed that the more degenerate members of the 

 group are the Petromyzontidae, and the degree of development of some 

 of the various members of the class ^) as deduced from other points in 

 their anatomy would be P. fluviatilis or planeri, P. marinus, Mordacia 

 mordax, Geotria ^), Myx'me glutinosa and Bdellostoma. 



The phylogenetic development is the reverse of this — that is, 

 the Myxinoids, Myxine and Bdellostoma, are the most primi- 

 tive forms, indeed the most primitive Vertebrates now 

 existing. 



1) A thorough comparative investigation of the anatomy of the 

 whole group of the Marsipobranchs is one of the greatest desiderata 

 of Vertebrate morphology. 



2) Thanks to Dr. Gùnïher's great kindness I was able to examine 

 the teeth of Geotria. They are Petromyzontoid. 



