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It would seem that the development of this character is dependent on the hormone from the mature testis, and in order to conform with the arguments used by me in other cases, the spinulation should have some definite function in relation to the habits of the sexes, and this function should involve some kind of external stimulation restricted to the mature male.
The shortening and spinulation of the scales are greatest in punctatus, least in norvegicus. In punctatus there are teeth on the vomer, in unimaculatus none, in norvegicus they are very small. If we consider fishes in general, we see that there is no evidence of any relation between many of the most important taxonomic characters and function or external conditions.
There is one point in which this sexual dimorphism in the Plaice appears to differ from typical cases, and which suggests that the greater spinulation of scales in the males has no function at all in the relations of the sexes, and is therefore not subject to and external stimulation.
On the east coast of North America the males in this form are more spinulated than the females; on the Alaskan coast, and apparently the Arctic coast, the females are spinulated, and the sexual difference in this respect is slight or absent. Lower salinity cannot be the cause of greater spinulation in this case, and thus it might be suggested that the condition was due to lower temperature.
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