No. 2.] MORPHOLOGY OF THE MYXINOIDEI. 22 1 



From a comparison of the above dental formulae, including in 

 all 248 specimens, we conclude : 



(i) That there is an exceedingly great variation in the num- 

 ber of the teeth, even more striking than in the case of the 

 gills. Thus these two characters (the number of teeth and 

 gills), the only two "constant" characters which Johannes 

 Miiller could find upon which to base his classification, are both 

 proven to be extremely variable. 



(2) In a large number of cases the two sides of the dental 

 plate are not symmetrical with regard to the number of teeth. 

 It is to be feared that the dental formulae given in systematic 

 accounts of Bdellostoma are, in many cases, based upon counts 

 of 07ie side only of the dental plate. 



(3) There is no constant relation between the number of 

 teeth and the number of gills. If there is any difference at all 

 worthy of note, the individuals with the larger number of gills 

 have a smaller proportion of teeth than might be expected. 



(4) There is no constant relation between the number of 

 teeth and the size or sex of the individual. The size and sex, 

 though not given in the above tables, were noted in every case. 

 While we should naturally expect that the larger individuals 

 would have a larger number of teeth, this is usually not the 

 case. In a 23-inch specimen, for example, which is considerably 

 above the average size, the dental formula was | [ ^^. 



(5) The outer rows of teeth have in a majority of cases 

 a greater number of teeth than the inner. In 312 cases the 

 teeth of the outer row were more numerous than those of the 

 corresponding inner row. In 178 cases they were equal. In 

 only 6 cases had the inner rows a greater number of teeth. 



(6) The dental formulae occurring oftenest are : -^^ I V" {^^)> 

 1^IH(38), HIH(32), HIU(22). It is evident that we 

 cannot speak of any one " typical " or predominant formula. 

 More than half the rows of teeth number 10 however, and in 

 nearly half the cases the corresponding outer and inner row 

 each contains 10 teeth. The number 9 is given next in fre- 

 quency, but occurs less than half as often as 10. More than 

 95/0 of the rows include either 9, 10, or 11 teeth. 



The Chilian specimens seem to average a larger number of 



