166 KAARE MÜNSTER STRØM. 



90. Tetmemorus lævis. (Kiitz.) Ralfs. 



As common as T. granulatus. Up to 4100 feet. 



91. Tetmemorus minutus. De Bary. 



Rare, only in two samples. A very minute form was 

 observed, the length of which was only 47 a, and the breadth 

 17,5 lii. 



Genus Euastrum. Ehrbg. 1832. 



92. Euastrum crassum. (Breb.) Kiitz. 



Rare, only a few specimens were observed. Size: long. 

 170 ^i, lat. 90 u. 



93. Euastrum pinnatum,. Ralfs. 

 General, but somewhat scarce. 



94. Euastrum, humerosum. Ralfs. 



This rare species occurred in one or two samples. 



95. Euastrum Didelta. (Turp.) Ralfs. 



Typical specimens were not common, although fairly 

 generally distributed. 



95 a. forma. 



Differs from the typical form in having: much more pro- 

 nounced lateral lobes, only three protuberances visible across 

 the cell, protuberances caused by an outside thickening of the 

 cell- wall. 



The large Euastra are very variable and therefore I can 

 not regard this form as a separate variety. The characters, 

 which I have pointed out are not sufficient to describe the 

 desmid as a distinct variety. The outline is very variable, and 

 in these large Euastra it is no good character. All the pro- 

 tuberances were very much reduced, and could only be observed 

 with extreme oblique illumination. In the vertical view there 

 are traces of the other two protuberances (a. in the figure.) Last, 

 the third character can not be regarded as good, as it is probable 

 that the cell as often develops the protuberances by an outside 

 thickening as by an inside. PI. V, Figs. 5 & 6. 



