74 PEOF. W. A. HBEDMAN ON THE OCCrTRKENCE OP 



shiftino- in bulk from the sand at the darker part of a tank to the end nearer 

 the window. This property accounts for the invariable occurrence of the 

 discoloured 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 (PI. 8. fio-. 5). There certainly seems to be a slight but definite cuticle 

 coverino- 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. 



Stao-es 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, so 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 I am not prepared to suggest. In the case of the Port Jackson 

 Glenodinium plague, Whitelegge thought the increase 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 I can find no previous record of such 

 an occurrence, it is probable that these swarms of Amphidinhim 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 l)rownish- 

 green patches on the sand between tide-marks. 



Postscript, Jime 5tJi, 1911. — Two days after reading the above paper 

 before the Linnean Society I was again on the beach at Port Erin. I found 

 in the same region what was apparently the same })atch of discoloured sand, 

 but on examining a scraping with the microscope found that the deposit was 

 now wholly composed of a golden-yellow Naviculoid Diatom — one of the 

 '"'' Ampldsbcena group '' of Navicula. I have searched the beach carefully 

 between tide-marks, and have examined samples from every suspected patch 

 of sand, but can find no trace of Ampliidinhim. 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 



