BRITISH DIATOMACEiE. 51 



the central and largest unarmed ; processes little exceeding the 

 median elevation in length ; papillate, rounded ; cellules of eleva- 

 tions distinct, those of valve and connecting-membrane minute. 

 Length of valve '0033" to -0058". 



Marine. Dredged off the Island of Skye by Mr. Bar lee ; communicated 

 by Prof. Williamson, Jan. 1852. 



In a letter from Prof. Williamson, he says that the present beautiful 

 species has been referred by Prof. Bailey to Zygoceros under the name of 

 Z. Tuomeyii. I have no means at hand of determining the point, nor of 

 ascertaining the identity of the American with the British species. 



Plate XL VI. 323. 



Genus 47. ISTHMIA, Ag. 



Frustules compressed, trapezoidal, one valve with a produced angle, 

 at which mucus is secreted, serving to retain the frustules after 

 self-division in an irregularly-branched filament; valves convex, or 

 subcylindrical ; S. V. elliptical. Structure cellulate ; cellules more 

 or less hexagonal. 



The mode in which the frustules cohere after self-division in the 

 present genus is quite peculiar; their attachment not being at 

 apposed angles, as throughout the subtribe, but indiscriminately to 

 any part of the adjacent frustule ; this gives a branch-like appearance 

 to the aggregated filament. The cause of this irregularity is quite 

 obscure. The little cushion of mucus, by which the frustules cohere, 

 is evidently produced subsequent to self-division, and the angle which 

 secretes this mucus is easily known by its more minute cellules and 

 less firmly siliceous membrane. The two British species which the 

 genus contains, have been so inextricably confounded together under 

 the common name of /. obliquata, that it appears expedient to adopt 

 new specific names for both ; characters for which, following Kiitzing, 

 I . find in the costae, so conspicuous in the one, but absent in the 

 other. The comparative length or breadth of the frustules, relied 

 upon by most writers, are characters wholly to be discarded, being 

 dependent upon the stage of growth or accidental circumstances, and 

 equally variable in both species. The synonymy, from the confusion 

 to which I have alluded, is very uncertain ; but I have endeavoured to 



e 2 



