106 BEITISH DESMIDIACE.E. 



isthmus, 23-40 /n ; breadth (maximum) of polar lobe 

 55-75^; thickness 55-60 ^ ; diam. of zygospore with- 

 out spines 80-116 //,, with spines 170-195 /A. 



ENGLAND. Cumberland! Westmoreland! (Rolfs). 

 AY. and N. Yorks ! Lancashire! Cheshire (Roy). 

 Leicestershire (Roy). Warwick (Wills). Gloucester- 

 shire (Rolfs). Wilts ! Surrey (zygospores from 

 Thursley Common) ! Sussex (Ralfs). Kent! (Ralfs). 

 Hants! (Rolfs). Devon! (Bennett). Cornwall! (Raffs); 

 zygospores from Halgavor Moor ! 



WALES. General ! Up to 2,200 ft. on Glyder Fach, 

 Carnarvonshire ! 



SCOTLAND. - General ! (Roy Bissett). Up to 

 8,500 feet on Lochnagar ! Scarce in the plankton ! 

 Outer Hebrides ! Shetlands ! 



IE ELAND. --General ! 



Geogr. Distribution. France. Germany. Austria. 

 Galicia. Hungary. Italy. Norway. Sweden. Denmark. 

 Bornholm. Finland. N., Central, and S. Russia. 

 Faeroes. Greenland. Japan. India. Sumatra. Aus- 

 tralia. Xew Zealand. United States. Brazil. 



M. denticulata is a common British species, and is more 

 often met with than any other species of the genus except 

 M. truncata, the latter being more plentiful at elevations of 

 1,000 feet and upwards. It can sometimes be obtained in 

 pure gatherings in small boggy ditches or at a boggy spring. 

 Such pure collections we have made at Eldwick and Adel in 

 W. Yorkshire. 



The ordinary form of M. denticulata is a little smaller than 

 M. rotata, the lateral lobes are more equal and equally lobed, 

 and their ultimate subdivisions are truncate-e margin ate and 

 not bidentate. The polar lobe is not exserted beyond the 

 periphery of cell as in M. rotata, is more cuneate in shape, 

 with a deeper median notch, and possesses rounded or 

 rounded-truncate angles. In the side-view of the semicell 

 M. rotata is stouter and more turgid than M. denticulata, and 

 in the vertical view it is also stouter with a solitary rounded 

 protuberance on each side, whereas M. denticulata is more 

 slender in vertical view and possesses three less-elevated pro- 

 tuberances at the middle on each side. 



