MARINE BIOLOGICAL STATION AT PORT ERIN. 71 



two weeks of September, when 22 species were noted. 

 Chaetoceras was represented by six species, which made 

 up the bulk of the plankton. C. decipiens was the most 

 common Diatom in the autumn maximum, and the 

 specimens were larger and in better condition than usual. 

 A gathering taken in the Bay with the coarse net on 26th 

 September contained over 26^ millions of Chaetoceras. 

 Of that number fully 25 millions were Chaetoceras 

 decipiens. A noteworthy swarm of Calami s occurred on 

 May 17th. A collection measuring 38"5 c.c. contained 

 48,000 adult Calanus. Diatoms were then very scarce. 

 It may be interesting to note that the swarm of Calanus 

 was preceded by a well-marked invasion of Dinoflagellates 

 which entered the Bay in May, just after the end of the 

 vernal Diatom maximum. The maximum of this 

 invasion appears to have been reached on the 9th May. 

 On that date the coarse and fine nets captured a total 

 quantity of 23 c.c. of plankton, aud the number of 

 Peridinians amounted to 8^ millions. 



It is interesting thus to find again, what we have 

 demonstrated before, namely, that Diatoms (see fig. 12), 

 Dinoflagellates (fig. 13), and Copepoda (fig. 15) succeed 

 one another in that order in the summer plankton of the 

 Irish Sea. The unusually large swarm of Peridinians 

 early in May followed close on the termination of the 

 great Diatom maximum in April, and was in its turn 

 succeeded by an unusually large swarm, amounting to 

 about 70,000 in one haul, of large Copepods on May 17th 

 and a few succeeding days. By the 20th of May the 

 large Copepods (Calanvs helgolandicus, fig. 14) had 

 disappeared, and their place was taken by quantities of 

 the more usual small summer Copepoda (fig. 15). At the 

 very end of May an invasion by the summer Diatom, 

 Rhizosolenia (fig. 16) attained unusual dimensions. 



