MARINE BIOLOGICAL STATION AT PORT ERIN. aT 
associated with me for some years in studying the 
plankton of the Irish Sea, remarked when I showed him 
some of the Phyto-plankton samples from Canna, Rum 
and Ardmore, ‘‘ If I had not seen the locality and date 
on the bottles I should have placed them without doubt 
as Irish Sea gatherings taken in April.’’ And the 
resemblance, I may add, is not merely in general 
appearance, but extends to the microscopic composition. 
The gatherings from Ardmore, for example, contain 
abundance of Chaetoceros contortum and C. decipiens, 
Rhizosolenia semispina, Lauderia borealis, Thalassiosira 
gravida and T. nordenskioldii—all of them Diatoms that 
are characteristic of an April gathering in the Irish Sea, 
off Port Erin. The abundance of the two species of 
Thalassiosira makes this and other July gatherings from 
round Canna and Mull quite unlike a September Diatom 
haul in the Irish Sea, as the genus Thalassvosira, 
abundant in the north, is practically absent at the time 
of the autumnal maximum in the south. 
We have probably not yet enough information before 
us to justify any attempt to explain the remarkable 
difference between the Hebridean plankton gatherings 
and those from the Irish Sea in summer. It may be that 
the great vernal maximum, which dies away in May or 
June in the Irish Sea, passes off more slowly further 
north, and is still found lingering on in some parts of 
the Hebrides until the end of July; or it may be that in 
some of these deep northern channels the Diatoms, that 
elsewhere constitute our vernal maximum, remain on in 
comparative abundance throughout the greater part of 
the year. <A third possible explanation is that the 
Diatoms constituting these July Phyto-plankton 
gatherings may have invaded the Hebridean seas from 
the North Atlantic at some period subsequent to the 
vernal maximum. 
