576 MR. J. A. MILNE ON THE 



fish. The anterior part is covered by a mass of concentric lines 

 or ridges, from the relative positions of which much may be 

 learned. The posterior part is almost without lines. This 

 formation is common to the scales of all members of the 

 genera Salmo and Oncorhpnchics. The whole scale is covered 

 by a membrane and grows with the fish. When the fish — and 

 with the fish, the scales — grows slowly the concentric lines, which 

 seem to be produced at a fairly uniform rate as to number, are 

 situated closely together ; when they grow quickly the lines are 

 placed further apart. When the fish does not grow at all, the 

 scales also cease to grow, and no lines are added to their surfaces. 

 The scales first appear upon young salmon as minute bony plates 

 under the skin. They may first be noticed when the fish is 

 about three months old, and about | inch in length (Yogt, 

 'Embryologie des Salmones,' 1842, Klaatsch 1890, and Dahl, 

 'Age and Growth of Salmon and Trout in Norway,' 1912). 

 When about four months old and 1^ inches long the little fish 

 may have from 2 to 5 rings round their scales ; and when about 

 eight months old, that is just before the winter comes on, there 

 may be, roughly speaking, from 8 to 20 lines already visible on 

 each of their scales. Within small limits the number of lines 

 varies in diflerent scales even from the same fish. We know that 

 the yearlings when kept in ponds feed well and grow rapidly 

 during the summer months, and that as winter approaches they 

 take less nourishment, finally at times fasting completely for 

 three or four days on end. We also know that their growth 

 progresses in proportion. This mode of life is clearly depicted 

 on their scales. Surrounding the nucleus, the lines formed in 

 summer are at some small distance from each other, and each 

 line can usually be traced right round the scale. The lines 

 formed in winter are much closer together and are usually more 

 numerous on the anterior portion of the scale. 



When the growth of the next year begins in the spring, the 

 lines first formed are again wider apart and generally continuous 

 all round the scale, and, later on, the winter formation of the 

 first year is repeated. Thus there is formed an area of well-spaced 

 lines followed by a band composed of lines very close together. 

 This band is usually most noticeable in front, and is followed by 

 another area of open lines and a second band which is probably not 

 quite so well defined as the first. The accompanying photograph 

 (text-fig. 97) of the central part of the scale of a Steelhead Trout 

 (^S*. gairdneri) well illustrates the formation. The line A points 

 to the completion of the first winter band, B of the second winter 

 band, C of the third, and the end of the line D shows the point 

 in the next year when the smolt left the i^ver and commenced to 

 grow rapidly in the sea. 



In the sea growth proceeds as before, but at a much increased 

 rate, and the difierence between fast summer and slower winter 

 growth continues to be apparent on the scales. 



Fish that are born, and spend all tlieir lives in the sea, show 



