MIC'RASTEKIAS. 77 



Zygospores globose, radially ornamented with stout 

 spines, either simple, bifid, trifid, or quadrifid at their 

 apices. 



This genus includes some of the handsomest of British 

 Desmids, many of the species being' remarkable for the 

 elegance of the tabulation of the semicells. There is always 

 an odd number of lobes to each semicell, the middle one 

 being differentiated from the rest as the polar Lobe. 



Many of the species are flattened and disc-shaped, the 

 cells beino- relatively much thinner than, in any other g'enus 



i_j tJ t/ O 



of Desmids. Certain species connect this genus both with 

 Euastrum and Xantlndliini. 



Micrasterias and Euastrum were both included by Kuntze 

 in his genus " Helierella," but we have previously mentioned 

 that this name cannot be used for a genus of Desmids. 



Species of this genus generally occur intermingled with 

 other Desmids, and some of them are only found in the very 

 richest localities for these plants, being confined to the lakes 

 and bog's of the Older Palaeozoic and Precambrian areas. 



O 



There are 18 British species, which can be arranged as 

 follows : 



SECTION A. (Holocystis Hassall.) Polar lobe entire ; lateral 

 lobes of seniicells two, transversely placed, generally entire, and 

 attenuated to their extremities. 



1. M. oscitans. 



2. M. pinnatifida. 



SECTION B. (Actinocystis Turner.) Polar lobe with a median 

 incision of variable depth (rarely almost absent) ; lateral lobes of 

 semicells four, radiately disposed, and widening outwards. 



a. Lateral lobes o-enerallv much divided : interlobular 



i 



incisions narrow. 



f Interlobular incisions not deep. 



3. M. truncata. 



4. M. crenatit. 



5. If. Jenneri. 



6. M. conferta. 

 ft Interlobular incisions deeper. 



7. M. papillifero. 



8. M. Murrayi 



9. M. Sol. 



10. M. apiculato. 



11. M. rotata. 



12. M. denticidata. 



13. M. verrucosa. 



14. M. Tliomasianii. 



