MICKASTEEIAS. 123 



a deep and "wide incision. There are thus three den- 

 ticulate processes to each lateral lobe, the lowermost 

 one being somewhat larger than the two upper ones. 



Dimensions of British specimens : length 181- 

 223 /x; breadth 153-188 /*,; maximum breadth of polar 

 lobe 88-115 ft ; breadth of isthmus 30-33 p. ; thickness 



Dimensions given by various authors :- -Length 132- 

 224 p; breadth 115-208/1. 



SCOTLAND. In the plankton of Loch nan Eun and 

 Loch Bhaic !, Perthshire; Lochs Euar ! and nan 

 Cuinne, Sutherland ; Lochs Burraland and Littlester, 

 Shetlands (./. Murray}. 



Geogr. Distribution. - - Germany. Sweden. India. 

 Banka Is. Samoa. Fiji (var.). New Zealand (var.). 



After a careful comparison of specimens of M. Maliabulesli- 

 warensis from divers parts of the world, it appears to us that 

 there is one variety,, which although in itself very variable, 

 yet retains certain distinctive features. This variety is dis- 

 tinguished by the subdivision of the superior lateral lobules, 

 so that there are three lateral processes instead of the two 

 which are found in the more typical forms. The relative 

 length, thickness, and divergence of these processes is ex- 

 ceedingly variable, but the presence of the Hire? -processes in. 

 place of two is a character the use of which is quite justifiable 

 as a varietal distinction. It was originally described as 

 Micrasterias Wallicliii by Grunow, but a comparison of its 

 different forms with those in which the superior lateral lobules 

 are undivided shows conclusively that it can only be con- 

 sidered as a variety of M. Mahabuleshwarensis. 



In the British Islands, M. Mahabuleshwarensis var. Wallicliii 

 has only been found in the plankton of certain Scottish lochs, 

 and the credit of this discovery rests entirely with Mr. James 

 Murray of the Scottish Lake Survey. In some cases the 

 specimens were subject to great variation, and this was 

 especially the case in Loch Bliaic, Perthshire. The variation 

 was principally in the duplication of the lateral processes. In 

 some individuals the upper process of the superior lateral 

 lobules was duplicated (vide PL LY, fig. 2), and in others the 

 lower process was similarly doubled. A few individuals 

 showed a duplication of the large processes of the polar lobe 

 (vide PI. LY, fig. 3). This curious abnormal state, with six 



