j^g HEFFERAX. [VOL. II. 



o 



4. Relation of Individual and Specific Variation. 



Out of fifty different species of Nereis which I found de- 

 scribed by various authors, the number of teeth was stated for 

 forty-seven. The numbers ranged from o in two cases, in 

 which the edentalous condition of the jaws was an important 

 specific character, to 20, given by Audouin and Milne-Edwards 

 ('29) for N. fucata. The number of teeth of TV 7 ", fucata is 

 given by Ehlers ('68), however, as 7, by Johnston ('65) as 5 to 

 10, and in the Challenger ('85) reports as 14 to 16. It would 

 be difficult here, as in the case of a few others, to decide 

 which observer came nearest to the modal condition of the 

 species. It is also impossible to tell whether they counted 

 the total number of teeth including those covered by a chiti- 

 nous band, or whether they referred only to the prominent 

 definite teeth. Ehlers makes the distinction only in N. vircns, 

 where he gives the definite teeth as 5 to 6, total as 10. St. 

 Joseph ('88, '98) notes in description of N. divcrsicolor that of 

 8 teeth 2 are indefinite, and in N. floridana that of 9 teeth 

 the lower 4 are buried in a clear translucent covering. For 

 these two species, respectively, Ehlers has given 8 and 9 teeth, 

 evidently counting both definite and indefinite. 



After attempting various methods of striking averages of 

 the statements made by different authors I finally decided to 

 use Ehlers' s numbers alone as most reliable, adding a few of 

 those given by St. Joseph in which there was less doubt that 

 the total number of teeth had been counted. Sedation of 

 twenty-two species gave the following results, total teeth. 



Classes ...5 6 7 8 9 10 n 12 

 Frequencies 115263 2 2 



CONSTANTS. 



A == 8.727 F : + 1.354 



a- - 1.838 curve - Type I 



/3i == 0.000090 s = 5.862 



/3 2 =- 2.323 skewness = + 0.00966 



Any conclusions which can be drawn from these results are 

 necessarily of doubtful value. It will be seen that the mean 



