270 Mr. J. Ralfs on the Diatomacese. 



XXXVI. — On the British Diatomacese. By John Ralfs, Esq., 

 M.R.C.S., Penzance*. 



[Continued from p. 111.] 

 [With a Plate.] 



The frustule in most of the Cymhellece consists of three portions. 

 The central one is continuous and surrounds the internal colour- 

 ing matter, and is generally compressed so as to have two sur- 

 faces and two ends. The lateral portions are most frequently 

 nearly flat, corresponding to the septa between the joints of the 

 ConfervecB ; but sometimes they are convex, and then appear on a 

 front view as in Achnanthes. In Isthmia they are still more de- 

 veloped, and being compressed, look as if they belonged to the 

 front of the frustule. 



Striae which may be present on either the central or lateral 

 parts commonly terminate in puncta or dots. On the central por- 

 tion they are longitudinal and continued round the ends, are in 

 general strongly marked, and often assist in distinguishing the 

 genera. On the lateral surfaces the striae are always transverse, 

 less distinct, and frequently numerous and crowded. Sometimes 

 they are very faintly marked, but the more powerful the micro- 

 scope used in the examination, the greater will be the number of 

 species found to possess them. On this account it is extremely 

 difficult to pronounce whether a species has them or not j and 

 therefore, although they may sometimes assist in the discrimina- 

 tion of species, their absence or presence will not sufficiently 

 distinguish genera. 



Isthmia, Ag. {Eh.) 



Filaments attached; frustules oblique, trapezoid, cellulose, co- 

 hering at the angles by small processes. 

 This genus is easily distinguished by its cellulose structure 

 from all the neighbouring genera, except Biddulphia and Amphi- 

 tetras, which agree with it in having the frustules reticulated ; 

 but differ inasmuch as their frustules are not oblique, and all the 

 angles are elongatedf. 



The frustules vary in form, but are always more or less oblique ; 

 there is a process at one of the lower angles of each by which it 

 coheres to the next, and in the basal frustule this process forms 

 the stipes by which the plant is attached to other Algae. The 

 frustules are turgid, and the reticulations of the central portion 



* Read before the Botanical Society of Edinburgh. 



f To the distinguished botanist M, Montague, I am indebted for a few 

 frustules of an Isthmia from Cayenne {Isthmia polymorpha, Mont.), which 

 in some degree connects this genus with Biddulphia. It is very minute ; 

 the reticulations are obscure, and the frustules are sometimes slightly pro- 

 duced at each of the four angles. 



