2 BRITISH DESMIDIACEJ1. 



The genus Euastrum is separated from the preceding genera 

 of the Cosmarieas by the relatively short, compressed cells. It 

 resembles Tetmemorus in the presence of the apical notch, but 

 differs in its short, compressed cells, with sinuate or lobed 

 margins and central protuberances (or tumours) . 



Euastrum includes a great variety of species, some of which 

 are transitional forms connecting it with other genera, such 

 as Cosmarium and Micrasterias. E. verrucosum is the only 

 British species connecting it with Micrasterias, but quite a 

 number of British species connect the genus with Cosmarium ; 

 such (for example) are, E. occidentale, E. montanum, 

 E. Cormibiense, and E. crispulum. 



Most of the species are lobed, some more than others, and 

 there is always an odd number of lobes. The unpaired lobe 

 at the apex of the semicell is always termed the polar lobe ; 

 the others are known as lateral lobes. 



The name "Helierella }: ' (Bory, ' Dictionnaire classique d'His- 

 toire naturelle/ 1826, vol. viii, p. 98) which was revived by 

 O. Kuntze (' Revis. gen. plant/ 1891, p. 897, etc.) is absolutely 

 useless as a generic name. It is impossible to establish a genus 

 on the remarks made by Bory, and Kuntze's genus Helierella 

 includes two well-defined and well-known genera of Desmids. 



There are 40 British species, less than half of which can be 

 considered as common. They can be conveniently arranged, 

 with due consideration of their relationships, as follows : 



SECTION A. Polar lobe with a distinct median notch, often deep 

 and linear. 



a. Cells generally large (length 50 205/i), polar lobe 

 short ; cell-wall usually punctate or scrobiculate ; 

 external angles of polar lobe rounded. 

 * Lateral margins lobed or sinuate. 



1. E. ventricosum. 



2. E. crassum. 



3. E. humerosum. 



4. E. pinnatum. 



5. E. oblongum. 



6. E. Didelta. 



7. E. affine. 



8. E. ampullaceum. 



9. E. sinuosum. 



10. E. Jenneri. 



11. E. aboense. 



12. E. inerme. 

 ** Lateral margins entire. 



13. E. cuneatum. 

 14 E. ansatum. 



15. E. obesum. 



16. E. pingue. 



