592 PROF. W. A. HERDMAN ON THE OCCURRENCE OF 
shifting in bulk from the sand at the darker part of a tank to the end nearer 
the window. This property aceounts for the invariable occurrence of the 
diseoloured sand on the surface only and never in the deeper layers. 
The published figures of this species are not very good, so a view of both 
dorsal and ventral surfaces, as seen under a high magnification, is given 
here (Pl. 18. fig. 5). There certainly seems to be a slight but definite cuticle 
covering the greater part of the surface, although this has been denied by 
some previous writers. The two characteristic Dinoflagellate grooves 
certainly join, as is stated by Calkins but not by other observers. 
The posterior flagellum which projects freely from the body is not difficult 
to see, but the anterior one which lies along the transverse groove is not so 
easy to demonstrate, and may differ a little in position and extent from what 
is shown in the figure. 
Stages in longitudinal fission were frequently seen, and that is probably 
the commonest method of reproduction. What appeared to be conjugation 
between two individuals was observed in one instance. 
It may be that this organism lives normally in small quantities, хо as not 
to be conspicuous, in some region of the sandy beach, or possibly in some 
special habitat beyond the beach, and that the present vast increase in numbers 
has been due to some unusual conjunction of circumstances ; but what these 
were Г am not prepared to suggest. In the case of the Port Jackson 
Glenodinium plague, Whitelegge thought the inerease may have been due 
to exceptional rainfall and calm weather; but the occurrence this spring at 
Port Erin was preceded by unusually dry, but rather stormy, weather. 
I am inclined to think that, although T can find no previous record of such 
an occurrence, it is probable that these swarms of Amphidinium have been 
seen before at Port Erin, and possibly elsewhere. I fancy I have seen the 
phenomenon myself in the past, and have supposed it to be due to swarms of 
Diatoms, which certainly do cause some of the yellowish-green and brownish- 
green patches on the sand between tide-marks. 
POSTSCRIPT, June 5th, 1911.—Two days after reading the above paper 
before the Linnean Society I was again on the beach at Port Erin. l| found 
in the same region what was apparently the same patch of discoloured sand, 
but on examining a seraping with the microscope found that the deposit was 
now wholly composed of a golden-yellow Naviculoid Diatom—one of the 
* Amphisbena group” of Navicula. T have searched the beach carefully 
between tide-marks, and have examined samples from every suspected patch 
of sand, but can find no trace of Amphidinium. The Navicula, which was 
present in April in very small quantity (see above), seems to have completely 
replaced the Dinoflagellate. We have probably still much to learn in regard 
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