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, is a curve of either Type I or Type IV, with a slight skewness in a negative direction 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 one of Type I could be made by discarding one extreme individual out of four hundred. (3) The number of teeth on the right jaw appears to be slightly more variable than that on the left. (4) The degree of correlation between the two jaws is, on the whole, rather high, 0.820. Correlation is closer between the indefinite than between the definite teeth. An inverse relation exists between the number of definite and the number of indefinite teeth on the same jaw, and also one between the number of definite teeth and the age of an animal. (5) The class range of teeth as given by the different species of the genus Nereis has a close agreement with the class range of N. limbata. Hence this one species offers the material for the modal condition of all species of the genus. (6) The results of observations of many specimens showing irregularities in teeth point to the conclusion that a process of wearing away of the definite teeth takes place in mature animals, and therefore that age comes in to help produce small number of teeth, but is not a large factor in causing variation. Only one author, St. Joseph, makes note of a difference between young and old specimens, the young having the greater number of teeth. Thus the statements made in regard to the number in many species in which only one animal or at most very few specimens were seen and described by their discoverers, are of little value as criterions of the specific condition. In conclusion, I wish to express my thanks to Professor Charles B. Davenport, who not only generously furnished the material for this investigation, but by his oversight and advice greatly facilitated the progress of the work.