Occasional Papers of the Museum of Zoology 13;. 



vive unusual conditions. Such catastrophes would, of course, 

 influence the shape of the graphs. 



Summary 



The length of the life cycle in Ichfhyoinyzon unicolor in the 

 region of the Thunder Bay River, Michigan, appears to be 

 seven years. This is a longer life cycle than has been recorded 

 for any other lamprey. 



The greatest increase in length takes place during the first 

 year. At the end of the first year and a quarter the larvae 

 have attained to about one-half their full-grown lengths. From 

 this time on the increase is uniform from year to year except 

 the last, when there is a slight increase over the normal. This 

 may be correlated with changes leading to metamorphosis. 



The mortality rate is greatest between the first and second 

 years. A suggestion is made that this may be a period of low 

 mortality correlated with internal changes leading to the estab- 

 lishment of sex. 



