7U Proceedings ot the Royal Irish Academy. 



the Ceratdum as varying so that, while all sizes may be met with at the same 

 time in the same waters, the number of the large forms steadily increases 

 from June to September. He also states that both three- and foui-homed 

 foiTus may be found together in the same lake. We should not like to draw 

 any definite conclusions as to the cause of the vaiiatious from Krause's results. 

 We do not agree that \-iscosity is the sole factor or even the most important 

 one in causing temporal variation in Ceratirmi. 



So far as our figures go, the size- variations are as follows : — 





Apical. 



Antapic (Ion 



gest). Total lengUi. 



Jan., Feb., Mar 



Practically absent from 



plankton. 





April 13, 



. -62.5 



•34.5 





1335 



May, . 



. -560 



•33 





1-196 



June. . 



. -540 



•32 





M37 



July 13, 



. -463 



•25 





1-097 



August 18, . 



"375 



•233 





■925 



Sept. 8, 



. -45 



•245 





1^03 



Oct. 21, 



. -415 



•25 





•98 



So far as we can determine the variation is not of great importance. 

 There is, however, no doubt whatever that the Ceratium are lui-ffesf when they 

 first appear, and there is a very steady and most marked i-eduction in size as 

 the water increases in temperature. There is practically no increase in size 

 as the temperature falls in autumn. It looks as if the change in size was 

 not exactly due to viscosity changes. Thus we see in Lough Neagh the 

 same seasonal vai-iations present, which agi-ee in most respects with those 

 discovered by Wesenberg Lund. In the late summer months almost all the 

 Ceratium are of the four-homed vaiiety. This fits in with the viscosity 

 theory ; but it may be due to other factor's, as we have suggested above. In 

 May most individuals possessed three horns, and in June the fourth hoi-n was 

 present, but very smalL 



EOTIFEEA. 



Perhaps the most classic example of temporal vai-iation is that of Anurea 

 cochlearis, so well worked out by Lauterboi^n. Lauterbom arranged the 

 variations ol^erved into series which all follow on from certain original 

 " sutepecies." The size of the animal is in inverse proportion to the height of 

 the water-temperature, and the winter forms ai*e characterized by long 

 posterior spines. 



A curious discrepancy appears in the literature. Lauterboi-n, whose 

 researches are stated by Wesenbeig Lund to have been generally accepted, 



