SCALE OBSERVATIONS OF THE SQUETEAGUE AND PIGFISH. 309 



annuli are found on scales taken from these parts which correspond exactly to those on 

 scajes taken from parts of the body subject to expansion. 



(4) The process of expansion and contraction could not produce the separable lam- 

 inae of which the annuli seem to be edges. 



It seems to be explained by differences of calcification. The inferior layer, in which 

 the laminae occur, is the secreted product of the floor of the scale pocket. As the fish 

 grows, both the scale pocket and its secreting floor increase in size proportionately. We 

 thus get a scale constantly increasing in size and thickness, the lower lamina of which is 

 always the newest part. This secretion on the inferior side is constantly being added 

 and calcifies much more slowly as it is pushed outward. If calcification should vary, 

 we would find layers of more and less calcification. In this case, if the scale were torn 

 forcibly, the less calcified part would yield while the more calcified parts would adhere, 

 giving the idea of separable laminae. This was actually done; in one case six laminae 

 were easily separated. The scale, according to this view, is a solid mass, and the 

 apparant layers are strata of slightly differing degrees of calcification. That mineral 

 metabolism, at least, in some marine animals is dependent on temperature is indicated by 

 investigations" in which it is shown that the magnesium content of crinoid skeletons is 

 higher in tropical than in colder latitudes. It is quite possible that the same variation 

 will be found in the calcium content of fish scales — not only in fishes from different 

 regions, but in the different lamince of the same scale. 



Polarized light. — The utility of polarized light in age determinations is twofold: 

 (i) When the scale is young — i. e., less than i year old — it is monorefringent to polarized 

 light; when more than i year old — i. e., consisting of more than one lamina — it is bire- 

 fringent (the writer was unable to verify this observation) ; ** (2) when used with a selenite 

 plate, the annuli stand out in colors different from those between. This is said to be 

 due to the scarcity of mineral salts in the vicinity of the annuli. The chief value of polar- 

 ized light is thus to bring out the annuli clearly in obscure cases. The refringency of light 

 is of little value, since it only differentiates fishes less than i year old from those more than 

 I year old, which usually in fishes so young is sufficiently indicated by the annuli alone. 



The selective action of picrocarmine stain. — Picrocarmine is the most satisfactory 

 selective stain for scales. Its value in age determinations depends, as shown by Carlet 

 ( 1 878) , on the selection of the carmine by the uncalcified parts. By it the outer lamina — 

 i. e., the youngest — which is deficient in mineral salts, is stained pink. The next inner 

 lamina is an orange, while the older completely calcified laminae stain yellow with the 

 picric acid. The edges of the several laminae up to three or four from the periphery 

 also stain pink. The value of this stain in age determinations, then, is twofold: (i) It 

 differentiates the last lamink, which is always the most difficult in old fishes; (2) it also 

 stains the edges of the laminae between this one and the first or second. Thus in most 

 cases the age of a fish 5 or 6 years old may be easily deciphered. 



The origins of the radii. — Like polarized light and picrocarmine stain, the radii are 

 only supplementary to the annuli as a means of age determinations. 



o Clarke. F. W.. and Wheeler, W. C: Composition of crinoid skeletons. Professional Paper 90-D, U. S. Geological Survey. 

 June 16, 1914. 



l>Carlet (1878). 



