Occasional Papers of the Museum of Zoology 11 



not be absolutely certain of the year to which a certain larva 

 belongs, since there is a great deal of intergradation due to 

 inequality of growth, but one should be quite nearly right by 

 taking the larvae represented in each hump of the curve as 

 the number of each year. These numbers and their equiva- 

 lents in percentages are as follows : 



Age Thunc 



3 months 21 indiv. or 9.1% 



1% years 58 



2M " 37 



3l4 " 30 



4% " 26 



s'A " 26 



6V4 " 31 



On plotting these figures the curves represented in Fig. 4 

 are obtained. Theoretically, the largest number of individuals 

 should have been obtained of larvae three months old, and yet 

 the graphs show that relatively few were obtained of this age 

 as compared with some of the older stages. There may be 

 several reasons for this. For one thing, the young larvae a're 

 very small and some of those landed may have escaped obser- 

 vation. It may be also that the young larvae remain nearer 

 the spawning grounds, among rocks and higher up the streams, 

 during the first summer, due to the absence of conditions, such 

 as floods, etc., which later would be operative in carrying them 

 down stream and into more favorable collecting places. 



The largest year-group of larvae obtained comprised those one 

 year old. There is a sudden drop in number during the second 

 year, and this is correlated with a sudden drop in the growth 

 increment curve during the same period. This might indi- 

 cate that this is a critical period in the life of the larvae, when 

 vitality, perhaps, is low on account of some internal change 

 of great importance. In another species of lamprey, Bnios- 



