746 t>«- J- BEARD, 



Nothing could have less of a glandular character than the mouth 

 of a Cyclostome, and the mouth of Myxine is not a sucking mouth 

 at all but a boring mouth ^). 



Whether a sucking mouth be primitive or not — a question which 

 there can be no hesitation in deciding in the negative — the mouth 

 of the Marsipobranchs is evidently something quite different from the 

 suckers of Lepidosteus &c. The discovery of true teeth in the Myxi- 

 noid fishes places a very different complexion on the question of the 

 nature of their mouth. 



Whether the teeth sit on mandibular or maxillary skeletal portions 

 or not has nothing to do with the evidence they afford of the gnatho- 

 stomatous characters of the Marsipobranchii. Their evidence throws 

 light in three directions. 



In the first place, W^illiamson (25) shewed long ago, and Hert- 

 wiG (10) more recently confirmed and extended the discoveries, that 

 the teeth of Vertebrata are derived from dermal scales, and hence the 

 ancestors of the Marsipobranchii must have possessed scales in their 

 skin. 



In the second place, unless we make a first and only exception 

 to the rule, true teeth postulate the existence at one time or another 

 of a true biting mouth, for originally a biting mouth is as necessary 

 to true teeth as they were primitively to every Vertebrate biting 

 mouth. Hence the ancestors of the Marsipobranchs must have pos- 

 sessed biting jaws, and the sucking mouth is a secondary development, 

 which is accompanied by a total or partial degeneration of the true 

 teeth. This degeneration is complete in the Petromyzontidae, incom- 

 plete in the Myxinoids. 



And lastly to reverse Balfour's argument and employ its con- 

 verse against his views: if they had true teeth they must have had 

 a true branchial skeleton, for true jaws are developed from a portion 

 of the branchial skeleton. 



The discovery of true teeth in the group only confirms Huxley's 

 view indirectly: by lending an a priori probability to the conclusion 

 that some remains of the true jaws are also present. I need hardly 

 remark that teeth occur on so many bones in the mouth of fishes, 

 that the presence of a tooth or teeth on a particular oral cartilage 

 in a Myxinoid does not of itself afford evidence of the homology of 



1) In the lingual apparatus of the Myxinoids Nature has tried to 

 make a weapon analogous to the radula of a Mollusc. 



