LIFE HISTORY OF LAKE HERRING OF LAKE HURON 271 



IRREGULARITIES IN SCALE STRUCTURE 



Various irregularities may occur in the structure of coregonid scales. 



1. The normally small, clear, well-defined focus may be replaced by an expanded 

 central area, devoid of circuli, rough or granular in appearance and irregular in out- 

 line, the relative size of which depends upon the recency of its formation. (Fig. 3). 

 Scales with such expanded centers are termed "regenerated scales," because they 

 have replaced those that are lost (Ryder 1884, Scott 1912, Creaser 1926).= 



2. The first circulus, that which limits the normal focus, may assiune various 

 characteristics. It may be complete and entirely separate from other circuli (figs. 6, 

 8, etc.); it may be incomplete and continuous with other circuli, forming a spiral (fig. 

 9), or incomplete, with one half missing, so that it resembles a horseshoe. 



3. Occasionally a condition is found that may be interpreted by assuming that 

 young scales sometimes become dislocated and rotate in their scale pockets. As later 

 growth is normal in direction, there results the appearance of a large scale with a 

 smaller one inset, the two with foci in different positions and main axes at different 

 angles. (Fig. 4.) 



4. Small scars or patches conuuouly found on scales presmnably are the repaired 

 injuries of what was formerly the margin of the scale. These scars, like the expanded 

 centers of regenerated scales, are irregular in form, granular in appearance, and entirely 

 devoid of circuli or annuli. (Fig. 5 near focus.) Other patches clear, clean-cut, and 

 with obscure circuli occur very rarely. (Fig. 5 at margin.) These apparently are 

 not repaired injuries, as the lower layers in these patches of the scale appear normal 

 with well-defined and typically formed annuli, continuous with those of adjacent areas. 



5. With respect to the annuli, various irregularities appear. Their circuli are 

 not always approximated. The divergence of the circuli is sometimes apparent in one 

 lateral field only. Approximated circuli often appear between two annuli, so as to 

 form "accessory annuli." Especially is this the case during the years with large 

 growth increments, when the scales show more clearly a retardation in growth at a 

 temporarily unfavorable tune. In the scales of some fish these "accessory annuli" 

 simulate the true annulus so closely as to make an accurate age determiuation impos- 

 sible. (Fig. 6.) In such cases the fish are discarded unless other more easily read 

 scales can be found on them. Very rarely two annuli form in the place of one (fig. 7), 

 so as to produce a wide, double annulus. This may be interpreted as due to a 

 resumption of growth following its retardation caused by some unusual circumstances 

 in the life of the individual, such as an injury, disease, starvation, etc. Still more 

 exceptional is the case in which an annulus fails to form on all the scales of the same 

 fish. The first annulus of the scale shown in Figure 8 is very distinct and ujiques- 

 tionably a true annulus. Figure 9 shows a scale of the same specmien, in which the 

 first annulus is less distinct but recognizable. A third scale (fig. 10) of this same fish 

 shows no trace of the first annulus. That all three scales actually belonged to the 

 same individual and that, therefore, no error occurred in technicjue, such as would 

 accidentally introduce a stray scale into the scale sample of the individual, can be 



' According to Taylor (1916), Dahl (1910) was the first to explain correctly the significance of abnormally large foci in scales^ 

 However, Reighard (1906) published a photograph of a regenerated scale of a whitefish and designated it as an "atypical" scale 

 "probably formed after the fish had grown to some size and in place of scales that had been lost [p. 51)." 



