14 r.KITISH DESMIDIACE2K. 



pelagicum, N. paradoxum var. li>n</'i]'if.^, N. faculiferutfa, and 

 Xantliiiluini antilopseum are frequent plankton-Desmids. 



A few are notably alpine or subalpine ; snch are 

 Staurastrum acarides, X. Kjellmanni, Cosmarium 

 cymatopleurum, G. microsphinctum , ('. cyclicum, etc. 



We have collected the following species at an eleva- 

 tion of 2,700 feet on Glyder Fawr," North Wales :- 

 Cylindrocystis crassa, Pemiim Digit HX, P. polymorphum, 

 T&tmemorus granulatus, T. Brebissonii, Closterium 

 Ehrenbergii, Pleurotaenium Trl>rc>il<i, Emtxfrirm hiiutle, 

 Cosmarium Cucurbitq,, (.'. Logiense, C. Ralfsii, Stauras- 

 trum KjcUminini, and Hyalotheca dissiliens. 



We have also found the following in material 

 collected for us by Mr. T. H. Burkhill at an elevation 

 of 3,500 feet on Lochnagar: Cylindrocystis Brebissonii, 

 Pcn'urm esiyuum forma major., Ti'tint'iiwrtis Iseris, T. 

 granulatus, Euastrum biiudc, E. lobulatum, Micrasterias 

 denticulata, Cosmarium, lfnm var. minor, (.'. bioculatum, 

 C. creitdtum, Staurastrum- er<tx/nn, S. Kjcllmannii, N. 

 muricatum, S. margaritac&um, and /S'. orbiculare var. 

 depressum. 



At a still higher elevation, on some of the moun- 

 tains of Perthshire, we have gathered a few species 

 such as Cosmarium cyclicum. 



Some of the most striking of British Desmids appear 

 to be confined to the rocky districts of the west coast ; 

 such are Pleurotaenium nodosnm, Thcid'unn undulatum, 

 Micrasterias f/irrnta, Euastrum ]>iii</ii' M , ^ti/n 

 elongatum, N. longispimim, N. Arctiscon, X. ( Vr 

 N. Ophiura, S. verticillatum, and N. 7Mvx///V;/x^ var. 

 I/imdellii. 



Some Desmids are absolutely cosmopolitan, occurring 

 from sea-level to the snow-line; such are Cylindro- 

 cystis Brebissonii, Tetmemorus granulatus, and T. Items. 

 In fact, the latter species is known from hot springs 

 in Iceland and the West Indies. 



One of the chief determining factors of the abund- 

 ance of Desmids in any locality is the nature of the 

 geological formation of the district. Limestone and 



