162 BRITISH DESMIDIACE^l. 



Length 88-1 10 ju; breadth 67-79^; breadth of isth- 

 mus 22-26 ft ; thickness 47-58 ju. 



ENGLAND.- -Cumberland ! Westmoreland! (Sissett). 

 Lancashire ! W. and 1ST. Yorks ! Cheshire (Roy). 

 Warwickshire! (Ralfs ; Wills). Surrey! Sussex! 

 Kent! Hants! Wilts! Devon! (Bennett). Corn- 

 wall! (Ralfs; Marquand). 



WALES.- -General in Carnarvonshire and Merioneth ! 



SCOTLAND. - -General ! (Roy $ Bis sett). Shetlands ! 

 Common in the Outer Hebrides ! 



IRELAND.- -Donegal ! Galway ! Kerry ! Dublin 

 and Wicklow (Archer). Down! Antrim! London- 



\ ./ 



derry ! Very rare in the plankton of the Kerry lakes ! 

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

 land. Austria and Galicia. Italy. Faeroes. Nor- 

 way. Sweden. Denmark. Finland. Russia. India. 



e/ 



United States. 



This characteristic species is generally distributed in the 

 8phagnwm-boga of the British Islands, occurring in association 

 with Cosmarium Cucurbita, C. margaritiferum, C. difficile, 

 Euastrum crassum, E. ansatum, Xanthidium armatum, Micras- 

 terias truncata, M. denticulata, and other species which thrive 

 amongst submerged Sphagnum. It is not very closely related 

 to any other British species, although in its granulation it is 

 without doubt nearest to C. margaritiferum. 



Up to the present it has been very imperfectly described. 

 The central part of each apex is destitute of granules, and 

 there is also a considerable reduction in the size of the granules 

 immediately surrounding this area. The actual smooth area, 

 and also the amount of reduction of the apical granules, 

 varies greatly in different individuals. We had described 

 the condition in which the maximum reduction occurs as 

 " forma erosa," but we now consider it impossible to draw 

 any line of demarcation between the forms of this species. 

 All the European specimens we have examined have the 

 reduced condition of the granules at the apices, and the cell- 

 wall is in all cases densely punctate between the granules. 

 Nordstedt/s " forma subtumida" is merely one of the ordinary 

 forms of the species. The sides of the vertical view are 

 invariably subtumid, and the thickness of the cell commonly 

 greater than that mentioned by Nordstedt for his form. 

 The large conical, almost papilliform granules are solid. 



