COSMARUTM. 27 



As it is impossible to draw an arbitrary line between the 



i/ 



larger and smaller forms, they have not been discriminated in 

 the following localities : 



ENGLAND. Cullingworth and Eoundhay Park, W. 

 Yorks ! The Washes, Sutton, Cambridgeshire ! 

 Sutton Park, Warwickshire (Wills) ! Middlesex 

 (pure gathering from Brent Reservoir) ! Surrey ! 

 Sussex (Ra/fo). Wilts ! Near St. Just, Cornwall 

 (al)undant) ! 



WALES.- -Dolgelly, Merionetl 



I 



SCOTLAND. Bridge of Don, Aberdeen ; Garrol, 

 Kincardine (Roy $' Bissett). 



I HE LAND.- -Lower Lake of Killarney, Kerry ! Dublin 

 and Wicklow (Archer). 



Gt.'o<./r. Distribution.- -France. Germany. Galicia in 

 Austria. Hungary. Italy. Roumania. Sweden. 



O i/ / 



Bornholm. S. Hussia. Nova Zembla. Spitsbergen. 

 Greenland. Ceylon. New Zealand. United States. 



C. biretum is by no means frequent, being found principally 

 in the marshes and ditches of low-lying districts. It often 

 occurs in great abundance among various species of Pot.a- 

 moyeton, and may sometimes be obtained almost pure in the 

 larg'e drains of the east of England. When occurring in 

 great quantity C. biretum generally exhibits many variations 

 in form. The divergence of the lateral margins varies very 

 much (consult figs. 2, 6, and 7 on PI. CI), with the result that 

 the semicells of some forms possess a much broader apex than 

 those of others. The conformation of the apex is also markedly 

 different in different individuals. In some it is straight or 

 but slightly convex, and may even be retuse in the middle, 

 whereas in others it is strikingly elevated, being very convex 

 and often truncate in the median part (PI. Cl, tigs. 4 and 7). 

 Such differences may, however, be found in the semicells of 

 the same individual. 



The protuberance on each side of the vertical view is also 

 a character which varies greatly. Sometimes it is very large, 

 and all specimens show some signs of it, although in certain 

 individuals it is scarcely appreciable. 



The granulation of C. biretum is one of its most charac- 

 teristic features. 



The three described forms var. intermedimu Wille, forma 

 grwnlandica Boldt, and forma subconxpersa Boldt do not 



