Remarks on Myctophum glaciale (Reinh.). 45 
and also probably attains a larger size, though of course this 
evidence is not conclusive of their mutual relationship. 
I have outlined above the changes in the hydrography of 
the Mediterranean which — as I see it — have taken place from 
the Glacial Epoch down to the present time. The invasion of 
North-Atlantic forms was followed by an invasion of Central- 
Atlantic forms and the differences shown at present by the 
Mediterranean forms, I think must be regarded as the result of 
adaptation to the special hydrographical conditions of this almost 
closed-in sea. This is especially the case as regards those forms 
which are sub-specifically or racially distinct from the Atlantic 
forms, while the origin of those Mediterranean forms, which 
must be regarded as good species, may date further back. 
Further investigations of the Mediterranean fish-fauna will pro- 
bably show that racial differences exist in many of the forms. 
which this sea has in common with the Atlantic. The prelimi- 
nary results of an investigation made by Taning (1918, table 
p. 22—23) on the variation of the number of vertebre in the 
Myctophidae tend to show this. Of 11 species of Myctophum 
(including the sub-genera Diaphus and Lampanyctus) the number 
of vertebre is lower in the Mediterranean in 7 species, in 3. 
species it almost coincides, while in 1 species it is higher than 
in the Atlantic.*) 
1) A comparative study of the invertebrate fauna of the Mediterranean: 
and the Atlantic probably will bring to light some more data in support of 
the relict theori. Thus I note that Lysholm.& Nordgaard (1921 p. 34) 
in a recent paper on the Copepoda collected on the cruise of the “Arm. 
Hansen” in the North-Atlantic in 1913 are of the opinion that the pelagic 
copepode Hemicalanus acutifrons is an “arctic” relict in the Mediterranean,. 
as they find it necessary to assume that the species must have both a Polar 
and a Mediterranean centre of distribution. 
