84 P.KITISH DESMIDIACEJE. 



Geoyr. Distribution. France. Germany. Austria. 

 G-alicia. Hungary. Italy. Norway. Sweden. Den- 

 mark. Bornholm. Finland. Poland (form). N. and 

 S. Russia. Faeroes. N. India. Sandwich Islands. 

 United States. Brazil. 



This is one of the most abundant species of the genus, 

 more especially in the Sphagnum-loogs and pools of upland 

 districts. In almost all mountainous areas this Desmid is 

 frequent from 800 to 2,500 ft., and it is very remarkable that 

 its zygospore has not yet been found. 



It is widely distributed throughout Europe and North 

 America, and some twenty varieties of it have been described 

 by different authors. Many of these so-called " varieties/' 

 however, have no claim to rank as such, as the species is a 

 very variable one, the two semicells of the same plant often 

 being widely different in appearance. The tabulation of the 

 lateral lobes varies very much, and forms obtained from 

 different localities rarely agree in the extent and disposition 

 of their lobulation. We have given figures of a number of 



tZ' CJ 



specimens, mostly from different localities, which show great 

 differences in the lobulation. 



Each lateral lobe is normally divided into two lobules, each 

 lobule being emarginate. Sometimes the angles of the lobule 

 are almost spinate, or they may be furnished with subcapitate 

 projections (vide PL XLII, fig. 7). Sometimes the lobules 

 are tridentate (this form having been named " var. tridentata" 

 by Bennett), but many specimens occur in which some of the 

 lobules are tridentate and others emarginate. This is shown 

 on PL XLII, fig. 1. The lateral incisions exhibit much 

 variation in the extent to which they are open or closed. 



The plant described as " Cosmarium quadragies-cuspidatum 3 

 by Corda was only a form of this species, the author (and 

 certain subsequent authors) having mistaken the radiating 

 fibrillar structure of the mucous investment for an armament 

 of spines. 



The form in which the lobules are furnished at each anQ-le 



O 



with a distinct spine is very rare in the British Islands. It 

 was first specially mentioned by Brebisson, being' named by 

 him " Euastrnm semiradiatum ' (vide Menegh. ( Synops. 

 Desm/ 1840, p. 215). Kiitzing (' Spec. Alg/ 1849, p. 170) 

 placed it as " Micrasterias semiradiata," and Cleve (' Sverig. 

 Desm.' 1864, p. 487) as " M. truncata var. semiradiata 33 

 Istvanffi subsequently (1887) created another synonym by 

 renaming the same form " M. truncata subsp. radiosa" ! 



