DEALFISHES. 
313 
at least until it is 100 mm. long (excluding the caudal 
fin), without any reduction of the long, anterior rays of 
the dorsal or the ventral tin (fig. 81, a). Sometimes, 
however, this reduction appears at an earlier period, the 
elongated, lower rays of the caudal tin also shrinking 
In the course of development, however, a great 
dissimilarity may prevail: the same stage of develop- 
ment is attained earlier (at a smaller size) by one larva 
and later by another; and we are still far from com- 
petent to decide with certainty whether this dissimilarity 
and vanishing, while the upper division of the caudal 
fin, which eventually assumes the appearance of a per- 
fect fin, with the two outermost rays strongest and long- 
est, takes up a vertical position on the dorsal edge of 
the end of the tail, in the form of an upright fan (fig. 82). 
depends on a difference of species. We know, however, 
that, at a more advanced age, great individual varia- 
tions appear in the characters of the best-known species. 
Emery has shown that, in the Mediterranean Trachy- 
ptems iris, the length of the head, during the growth 
Scandinavian Fishes. 
40 
