120 BRITISH 



and slightly undulate, accessory processes reduced and 

 rounded ; incision between the two lateral lobes nar- 

 rowly linear. 



Length 117 /i,; breadth 98 p,; breadth (max.) of 

 polar lobe 55 ju, ; breadth of isthmus 19 JJL. 



ENGLAND. Mickle Fell, N. Yorks ! 



WALES. Near Dolbadarn Castle, Carnarvonshire ! 



Geogr. Distribution. United States. 



Var. Lewisiana West. (PI. LIV, figs. 5, 6.) 



M. Americana var. Lewisiana West, Alg. N. Wales, 1890, p. 286, t. 5, f . 13 ; 

 West & G. S. West, Some Desm. U.S. 1898, p. 296. 



Smaller than the type ; polar lobe shorter and 

 stouter, lateral angles short and thick, with tricrenate 

 apices, apical margin almost straight, very slightly 

 retuse in the middle, accessory processes wanting, or 

 represented by very small, rounded protuberances ; 

 lateral lobes with the margins crenate (not dentate). 

 Cell-wall distinctly punctate. 



Length 95-100 ^ ; breadth 72-85 ju, ; maximum 

 breadth of polar lobe 48-54 /x; breadth of isthmus 

 16-17-5 p.. 



WALES. Llyn Padarn, Carnarvonshire ! 



Geogr. Distribution. United States. 



This characteristic variety can be recognized at once by 

 the form of the polar lobe. The dimensions of tlie American 

 specimens were : length 80-103 ju ; breadth 70-84 ju; breadth 

 of isthmus 23-26 fj.. 



Var. Boldtii Gutw. (PL LIII, fig. 6.) 



M. Americana forma, Boldt, Desmid. Gronland, 1888, p. 5, t. 1, f. 1. 

 M . Americana var. Boldtii Gutw. Walir. d. Prioritat, 1890, p. 73 ; Flor. 

 Glon. Okolic Lwowa, 1891, p. 74, t. 3, f. 27. 



Polar lobe shorter than in the type, lateral angles 

 produced into very short processes, apical margin very 

 slightly concave, accessory processes much reduced, 

 sometimes wanting on one or both semicells ; lateral 

 lobes more irregularly dentate. 



Length 110-137 p; breadth 9 6-1 11 ^; maximum 



