142 HEFFERAN. [VOL. II. 



one of Type I could be made by discarding one extreme indi- 

 vidual 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 ani- 

 mals, 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 ani- 

 mal or at most very few specimens were seen and described 

 by their discoverers, are of little value as criterions of the spe- 

 cific 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. 



