108 BRITISH DESMIDIACE.E. 



the lakes) and Llyn Idwal, Carnarvonshire ! Llyn 

 Coron, Anglesey ! 



SCOTLAND.- -General ! (Itoij fy Bissett). Lewis and 

 Harris, Outer Hebrides ! Orkneys ! Shetlands ! Scarce 

 in the plankton ! 



IRELAND.- -Donegal ! Armagh ! Dublin and Wick- 

 low (ArcUer). Gralway ! Kerry! Cork! 



Geogr. Distribution.- -France. Germany. Austria. 

 Hungary. Italy. Norway. Sweden. Denmark. 

 Bornholm. Finland. N". and S. Russia. Greenland. 

 Nova Zembla. Central China. Japan. India. 

 New Zealand. Abyssinia. Central and E. Africa. 

 United States. Brazil. Paraguay. 



This most characteristic species exhibits considerable varia- 

 tion in the degree of development of the spines,, and the 

 semicells are also slightly variable in form. These points 

 can be noticed by an inspection of figs. 4-12 on PL CXVL 



Examples are often found in which the spines of one semi- 

 cell are normal and those of the other reduced or absent, and 

 by further division of such a plant a specimen may be 

 produced in which the spines are entirely absent (vide 

 PI. CXVI, fig. 10). On the division of this spineless cell, 

 however, the newly-formed semicells may, and usually do, 

 develop typical spines. Thus, a character which may be 

 entirely lost in a few individuals, as a result of repeated 

 vegetative division, is frequently reproduced at its maximum 

 in a succeeding division (Consult PL CXVI, fig. 11, and the 

 remarks under Cosmarium Regnesi in Vol. Ill, p. 38). 



The production of occasional spineless individuals of this 

 Desmid leads one to the conclusion that Arthrodesmus con- 

 vergent originated in the first instance from a Cosmarium of 

 the nature of C. depressum (Nag.) Lund. In fact, the 

 resemblance between A. convergens and Cosm. depressum is 

 so very close that many suggestions have been made as to 

 their specific identity. These suggestions are, however, 

 based upon insufficient knowledge of these Desmids. It 

 must be remembered that the vegetative divisions of Cos- 

 marium depressnm always produce unarmed cells similar to 

 the parent generation, but that the spineless cells of A. 

 convergens are nevertheless truly A. convergens with all those 

 inherited characters which will, probably result in the pro- 

 duction of full-grown typical spines in the new semicells of 

 the next division. 



