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 fmstule in most of the Cymbellece consists of three portions. 

 The central one is continuous and siuTOunds the internal colour- 

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

 faces and two ends. The lateral portions are most fi'equently 

 nearly flat, con-esponding to the septa between the joints of the 

 Confa'vea ; but sometimes they are convex, and then appear on a 

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

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

 fi'ont of the frustule. 



Strise which may be present on either the central or lateral 

 parts commonly tenninate 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 sm-faces the strise are always transverse, 

 less distinct, and fi-equently numerous and crowded. Sometimes 

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

 scope used in the examination, the gi*eater will be the number of 

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

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

 therefore, although they may sometimes assist in the discrimina- 

 tion of species, their absence or presence will not sufliciently 

 distinguish genera. 



Isthmia, Ag. {Eh.) 

 Filaments attached; fiaistules obhque, trapezoid, cellulose, co- 

 hering at the angles by small processes. 



This genus is easily distinguished by its cellulose structm'e 

 from all the neighbom'ing genera, except Biddulphia and Amphi- 

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

 but difler inasmuch as theii* frustules are not obhque, 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 fi-ustule this process forms 

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

 frustules are tm'gid, 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 

 fi-ustules of an Isthmia from Cayenne (Isthmia polymorpha, Mont.), which 

 in some degi-ee 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. 



