112 BRITISH DESHIDIACE.E. 



Yorks ! Pilmoor and Terrington, X. Yorks ! Near 

 Cockley Beck, Lancashire ! Chobham and Thursley 

 Commons, Surrey ! 



SCOTLAND. Ross, Aberdeen, Kincardine, Forfar, 

 Perth!, Stirling, Dumbarton (Roy fy Bissett). Suther- 

 land ! Balallan, Lewis, Outer Hebrides ! Zygospore 

 from Fyvie, Aberdeen (Joshua). 



IRELAND. Louerh. Akibbon, Donegal ! Glen Caraerh 



O < ) o 



and Cloonee Lough, Kerry ! Dublin and Wicklow 

 (Archer). 



Geogr. Distribution. France. Galicia in Austria. 

 Norway. Sweden. Ceylon. Singapore. Java(var.). 

 United States. 



This species, which in its type form is so characteristic,, 

 is one of the most variable in the genus. The relative size of 

 the basal projections, and the number and disposition of the 

 surface-teeth are very variable, and it is sometimes difficult 

 to obtain two individuals which agree in these respects. 



M. Thomasiana has a greater affinity with M. denticulata 

 than with M. rotata, and some authors have placed it as a 

 form of that species. It is smaller than M. denticulata, the 

 tabulation is more acute, and it differs in a striking manner 

 in the projections present at the base of the semicells, as well 

 as in the teeth within the bases of the lobes and lobules. It 

 must be borne in mind, however, that these characters are 

 not constant. The teeth within the lobules are generally 

 reduced in number and somewhat irregular in their disposi- 

 tion, and it is not unusual for the median basal projection to 

 be entirely absent. Such forms are figured in Gr. S. West, 

 ' Variation Desm. J 1899, t. 9, figs. 6 and 7. 



Individuals are frequently met with in which the basal 

 projections are much reduced, and the surf ace -teeth are 

 scarcely evident. These constitute an intermediate series of 

 forms between M. denticulata and M. Thomasiana. Such a 

 form is figured on PL LI, fig. 7. 



Forma major "West. 



M. Thomasiana forma major West, Alg. W. Ireland, 1892, p. 134. 



Cells about half as large again as the type. 

 Length 315 ^u; breadth 285 /n ; breadth of isthmus 

 38 /u. 



IRELAND. Arclerry Lough, G-alway ! 



