No. 3] VARIATION I. .V THE TEETH OF NEREIS. 141 



of an excess of teeth at the base of a jaw on which some of 

 the extreme teeth had been lost, but I found no other indica- 

 tion of such regulation. Another individual presented a par- 

 tial right jaw, Fig. 8, which was a stump of about half the 

 length of the left jaw. The normal jaw was dark brown,- 

 almost black, while the stump was light straw color character- 

 istic of a young jaw or of the very base or imbedded part of 

 an old one. The color indicated new growth or regeneration, 

 in which case one would expect to find a production of small 

 indefinite teeth crowded at the base, as in the specimen of 

 Fig. 7, if that could be interpreted as a regenerative process. 

 On the contrary, the stump had exactly the same number of 

 teeth similarly disposed as the part of the opposite jaw which 

 corresponded to it. It may have been, then, only the rounded 

 stump of a broken jaw, although this explanation does not 

 account for the peculiar color. 



Abnormalities in other parts of the animals were looked for 

 only incidentally. I found no cases of double pairs of caudal 

 cirri, but all of the worms were not examined for this not 

 unusual abnormality, since the posterior parts of many of 

 them were not preserved. 



Fig. 9 shows a case of an abnormal segment. The fifteenth 

 segment extended only a little more than halfway across 

 towards the left side of the body, leaving one broad segment 

 on the left side in place of two, and a partially double para- 

 pod. The axis of the body was bent at that point, compen- 

 sation being made gradually by the greater width on the left 

 side of the segments immediately preceding and following. 



6. Summary. 



The results of this study may now be summed up as follows : 



(1) The typical condition for the total number of teeth of 

 N. limbata of Cold Spring Harbor, 1899, i g a curve of either 

 Type I or Type IV, with a slight skewness in a negative direc- 

 tion from the mode, 10. 



(2) In case of the calculation of the right total teeth, a 

 transition from a curve of Type IV to an equally serviceable 



