198 BRITISH DESMIDIACE^E. 



Ogwen, and Llyn Idwal, Carnarvonshire ! Llyn 

 Coron, Anglesey ! 



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SCOTLAND. Near Tillypronie. Aberdeen (Roy fy 

 Bissett}. Glen Shee, Perth ! Ben Laoigh, Argyll ! 

 Eenfrew ! Sutherland ! Caithness ! Skye in Inver- 

 ness ! Lewis, Harris, and X. Uist, Outer Hebrides ! 

 Orkneys ! Shetlands ! 



IRELAND.- -Donegal ! Xear Westport, Mayo ! Gal- 

 way ! Kerry ! Louth ! Armagh ! Down (at 2000 



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ft.) ! Wicklow ! and Dublin (Archer). Lough Neagh ! 

 Geogr. Distribution.- -France. Germany. Switzer- 

 land. Austria. Norway. Sweden. Faeroes. Fin- 

 land. Russia. E. Africa. 



C. pr&morsum is widely distributed in the British Islands, 

 more especially in marshes, and at the weedy margins of large 

 ponds and lakes. It sometimes occurs in bogs, but is only 

 rarely met with among Sphagnum. 



Owing to the imperfect nature of Brebisson's original 

 description and figure of this species much confusion exists 

 as to its exact identity. The Desmid which we have always 

 identified with C. przemorsum Breb. is a close ally of C. 

 margaritiferum, but differs markedly in the character of its 

 granulation. The apex is either perfectly smooth or with 

 but few reduced granules, and the marginal granules of each 

 side are very prominent. The granulation is entirely without 

 any definite arrangement and is generally most irregular. 

 The granules vary much in size, but there are always some 

 large ones in the centre of the semicells, sometimes forming 

 a more or less isolated group. C. pr&morsum differs from 

 C. margaritiferum in the more rounded basal angles of the 

 semicells, in the much greater irregularity of the granulation. 



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and in the great variability in size of the granules. There 



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are also fewer granules on the whole cell, and generally there 

 is a smooth area of variable extent on either side of the 

 isthmus. There are certain forms which appear to be some- 

 what intermediate in character between these two species, 

 and it is sometimes a matter of considerable difficulty to 

 correctly place them. The species have different habitats, 

 and it would appear from the immature zygospore of C. 

 prfe-morsum observed from among Scirpus fluitans on Keston 

 Common, Kent, that the zygospores of these two species are 

 markedly different. 



