126 Part III. — Twenty-third Annual Report 



skin, and I have not met with spinulated scales hitherto among those I 

 have examined for indications of age. 



Eeibisch therefore turned his attention, at Hensen's suggestion, to the 

 otoliths. He describes the appearance of these structures as seen by 

 transmitted light. He states that the first year's deposit consists of a 

 very dark, i.e. opaque, nucleus or kernel; this is surrounded by a narrow 

 transparent ring, then follows a broad dark zone, which is again sur- 

 rounded by a light zone, and this again is bounded by a dark contour. 

 He states that the nucleus and the first clear ring with part of the dark 

 zone are formed during pelagic life, the outer clear zone during the 

 sojourn in shallow water near the coast, while the dark contour is formed 

 only when the fish has migrated into deeper water, about January or 

 February. It will be seen that my results agree closely with those of 

 Reibisch, except in the last point, for I have not noticed that the dark 

 zone of the second year had begun to appear in specimens collected in 

 March and April, and it seems to me that it is formed in summer. 

 Reibisch figures the otolith of a specimen 11 '5cm. long taken at the end 

 of February, in which he believes the deposit of the second year had 

 begun and was visible at the anterior and lower side of the otolith. He 

 figures also an otolith from a specimen 16 '5cm. long taken at the end of 

 February which similarly shows the beginning of the third year's deposit, 

 and another from a specimen 23*5cm. which shows three complete years 

 and the beginning of the fourth. This specimen was a ripe male taken 

 on the 9th March, and the deposit of the fourth year in the figure is 

 almost as wide in some parts as that of the third. It seems tome difficult 

 to believe that this could have been formed in a few weeks, and more 

 probable that it represents the whole deposit of the previous year, so that 

 the specimen was four years old. It is in cases of this kind that the 

 difficulties of the method arise, and they can only be settled by ascertain- 

 ing with certainty at what time of the year the boundary line between the 

 annual zones is formed. Reibisch assumes that the new opaque deposit 

 begins in January or February, while my own opinion at present is that 

 it does not commence till much later. 



Reibisch concludes from his investigations that sexual maturity occurs 

 always at the end of the third year, and that the reason why the fish are 

 so different in size and weight at this period of life is that they were 

 hatched at different periods of the same season. In my experiments on 

 the rearing of flounders in captivity at Plymouth I also found that the 

 majority began to spawn at the end of their third year, but a few were 

 ripe at two years of age. 



Reibisch concludes that the darker layers in the deposit of one year 

 in the otolith corresponds to the lower temperature of the water in which 

 the plaice lives, and the more transparent layers to the warmer tempera- 

 ture. He also states that the excretion of carbonate of lime is weaker in 

 the first half of the year, when the temperature is low. According to his 

 reasoning therefore, the more opaque layers are those in which the 

 proportion of carbonate of lime is least, and these are formed at the time 

 when the lower temperature of the surface penetrates to the deeper w r ater, 

 which is usually from January to March, The annual period indicated 

 by a complete zone in the otolith would, on this view, coincide with the 

 calendar year and commence in January. 



My conclusions, from my own observations so far as they have gone, 

 are not in harmony with those of Reibisch on the above points. In the 

 first place, it seems to me probable that the opacity would increase, not 

 diminish, in proportion to the amount of carbonate of lime present : this 

 is certainly the case in bone and calcified cartilage, and it is also the case 

 in the scales, where the radiating and concentric lines let ween the sclerites 



