202 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



in Luce Bay form a group varying " racially " from that 

 of the Irish Sea, and that the growth-rate may be a 

 slightly different one. But to demonstrate this biometrical 

 examination of certain selected morphological characters 

 both in Irish Sea and Luce Bay plaice would be necessary. 



(4<) Growth and Age-Groups. 



About 16,000 plaice have been examined in the 

 laboratory during the years 1909-1913. As a rule, these 

 fish were received in samples of about 100 specimens. On 

 receipt, they were at once sorted into centimetre groups, 

 all the fish in each group being put into wooden trays. 

 The contents of each tray were then weighed to the nearest 

 gram, and the average weight of the fish in each group — 

 that is, the fish with a mean length of n-b cms. was calcu- 

 lated. The fish were then examined for age and sex. Sex 

 was determined by holding up each fish against a strong 

 light with the coloured side turned towards the observer : 

 only very infrequently was it necessary to make a dis- 

 section. The otoliths were then taken out and " read," 

 and the age-groups recorded. In the case of plaice of 

 less than 15 cms. in length, the sex was not, as a rule, 

 determined. When the sample was a large one, these 

 small fishes were sorted as usual, and then all the otoliths 

 were placed in groups on a piece of black cardboard, and 

 allowed to dry, so that they became attached. The cards 

 were then placed in weak glycerine, so that the otoliths 

 became slightly translucent, when the age-groups were 

 read, usually with the assistance of a hand-lens. 



It has been assumed that the method of age-determi- 

 nation by inspection of the otoliths gives reliable results. 

 The matter is not, however, so simple as it appears, and 

 there are perhaps some details with regard to which investi- 

 gation is necessary. The statistical method used by 



