NORTH SEA INVESTIGATIONS. 199 
observations recorded above, is that the diagnosis of Pl. platessa 
must be so amended as to allow for the occurrence of ciliated scales, 
especially in males. 
Judging by the analogy of fresh-water fish (e.g. Salmo, &c.), it 
seems quite possible that fish which reach a large size in the North 
Sea might remain permanently stunted if confined to the Baltic. 
On such an hypothesis one might regard the small mature Baltic plaice 
as distinct in nothing, save environment, from their larger brethren 
in the North Sea. The very general ciliation of the males in the 
former, and the rarity of this character in the latter seems, however, 
to show that a more important distinction exists, and that the small 
fish are a true variety, indistinct enough, no doubt, if specimens 
from intermediate localities could be procured. Living ova, which I 
took from some of the specimens described above, measured from 
1-706 to 1:796 mm. in the unfertilized condition, dimensions which 
overlap the extremes recorded for the ova of Atlantic plaice, but 
which nevertheless yield a considerably less mean diameter. That 
the Baltic herring are considerably smaller than the North Sea and 
Atlantic representatives of the same species is well known, and it 
would appear that the difference in size is apparent from the earliest 
stages, since the former, when newly hatched, are only 5°2 to 5°3 mm. 
long (Kupffer), and therefore about 2 mm. shorter than such newly 
hatched North Sea larve as have come under my own notice (Ann. 
Mag. Nat. Hist., 1889, p. 369). 
On the other hand, the flounders which I found amongst the con- 
signment of Baltic plaice were fully as large as those met with on 
our own coasts, and it is a significant fact that the flounder is a fish 
which flourishes best in brackish or even in nearly fresh water. The 
low specific gravity of the Baltic water is familiar to everybody, as 
is also the fact that its existing fauna differs, in the absence of 
certain marine organisms, from that of the open sea. Hence it is 
very interesting to note that these small plaice appear to reach a 
limit of size (about 13} inches) which practically corresponds to 
the size which is attained by a young North Sea plaice before it 
leaves such an estuary as the Humber for the offshore grounds. It 
is, no doubt, the quest of food suitable to its imcreasing needs that 
causes the emigration of the young North Sea fish, and even without 
special knowledge of the food-supply in the Baltic, it is perhaps per- 
missible to assume that it is the inadequacy of the food that limits 
the growth of the goldbutt. It is restricted, in fact, to a perma- 
nently estuarine condition. 
The question of dermal armature in relation to environment is 
much more difficult, and I am not at present prepared to attack it ; 
I may say, however, that materials which I have been accumulating 
