92 r.RITISH DESMIPTACEJE. 



SCOTLAND.- -Sutherland ! Inverness! Perth! Ayr! 

 Lewis and Harris, Outer Hebrides ! 



IRELAND.- -Donegal ! Gahvay! Mayo! Kerry! Cork! 



Geoyr. Distribution. (The following distribution 

 most certainly includes the records of forma minor, 

 var. indentatns, and yar. Italfsii). France. Belgium. 

 Germany. Austria. Italy. Norway. S weden. Den- 

 mark. Bornholm. Finland. Poland. S. Russia. 

 Fgeroes. Iceland. Greenland. India. Java. New 

 Zealand. Australia. United States. Cuba (var.). "W. 

 Indies (var.). Guiana. Brazil. Paraguay. Patagonia, 



Artlirodesinu* lucnx lias numerous varieties, some of which 

 are distributed all over the world. It is very probable that 

 many of the records given above refer to one or more of the 

 following varieties and not to what we consider the type 

 form. We have regarded the form described nbove as the 

 type form for two reasons. In the first place, most of 

 the forms of this Desmid are fairly stout with lono- diverg-ent 



/ o o 



spines ; and secondly, this form is nearest to certain forms 

 of Staurastrum from which it is probable that Arthrodesmus 

 Incus originated. It is, so far as we can judge, the least 

 specialized of all the forms of A. Incus. 



Some of the lono--spined forms with distinctly triangular 

 semicells appear to form a graduated series with the biradiate 

 forms of Staurastrum jaculiferum, and other species of Arthru- 

 desmus are closely related to certain of the triangular spiny 

 species of Staurastruui. In fact, there seems little doubt 

 th?it the majority of the species of Arthrodesmus have arisen 

 from certain species of Staiirastrum by the adoption of a 

 permanent biradiate character. 



In certain individuals w 7 e have noticed that through the 

 larger pores in the cell-wall a tough mucilage is excreted, 

 which assumes the form of blunt or emarginate processes 

 standing 1 out at rio-lit angles to the surface of the wall and 



O c_- O 



disposed one over each pore. These mucilaginous processes 

 occasionally turn yellow or even brown in colour and become 

 quite hard. (Consult PI. CXIII, fig. 15; also PI. CX1Y, 

 fig. 5 c.) 



Forma minor nob. (PI. CXIII, figs. 10-19.) 



Arthrodesmus Incus of very many authors. 



Cells not much more than half the size of the type, 



