94 BRITISH DESMIDIACE^E. 



with the incisions between the lobes widely open 

 and subradial; polar lobe with subparallel sides, 

 suddenly dilated near the apex, which is retusely 

 emarginate, angles emarginate- dentate, with a minute 

 tooth within the apical margin towards each angle ; 

 lateral lobes equal, divided into four equal lobules, the 

 median incision being twice the depth of the other two 

 incisions, each lobule emarginate-dentate. Cells with 

 a series of minute teeth or denticulations on each side 

 of the interlobular incisions and the sinus. 



Zygospore unknown. 



Length 142-151 /A; breadth 142-147 p; breadth of 

 isthmus 18 jut. 



SCOTLAND. In the plankton of Loch Ruar, Suther- 

 land ! (/. Murray). 



This species occurred in abundance in the plankton of Loch 

 Ruar, the only locality from which it has yet been obtained. 

 Its characters are very constant and do not agree with those 

 of any other described species of the genus. The incisions 

 between the lobes and lobules are all widely open with 

 concave sides, which feature causes them to be widest about 

 the middle. The sinus, which is more open than that of any 

 other incised Micrasterias, is one of the most striking charac- 

 ters of the species. 



The only species with which it could be confounded are 

 M. papillifera Breb. and M. Sol (Ehrenb.) Kiitz. (particularly 

 var. ornata Nordst.). From M. papillifera it is distinguished 

 by the widely-open sinus and incisions, which are also deeper, 

 and by the very different form of the polar lobe. It is dis- 

 tinguished from M. Sol by the widely-open sinus and incisions, 

 which are not so deep, and by the absence of the further 

 subdivision of the superior lateral lobes. 



M. Miirrayi possesses a series of minute denticulations 

 along the margins of the sinus and each of the incisions 

 between the lobes. This character is present in M. papillifera 

 and in M. Sol var. ornata. 



Var. triquetra West & G. S. West. (PI. XLV, fig. 4.) 



M. Miirrayi var. triquetra West & G. S. West, Scott. Freshw. Plankton, I, 

 1903, p. 538, t. 15, f. 3. 



Cells triquetrous, triradiate in the vertical view. 



