460 Mr. J. Ralfs on the British species of Gomphonema. 



tules are terminal. They are generally simple or binate; I have 

 never seen more than six frustules from the same point, and even 

 when more than two frustules are thus occasionally combined, 

 they are usually in contact only at the base; on most of the 

 branches also the frustules are only binate. 



In Licmophora the stipes is irregularly branched, each branch 

 being terminated by several frustules, often ten or more, united 

 by their sides into a fan-like form. Besides these terminal clus- 

 ters, the main trunks of the stipes have binate or ternate frustules, 

 sessile or nearly so along their margins, and either opposite or 

 alternate. 



The species of Gomphonema are commonly found in fresh water. 

 The frustules are cuneate, and at their upper margin have two 

 notches which look like puncta; as these are in some species 

 very evident and in others scarcely discernible, they assist in 

 forming the specific characters. 



There are two obscure puncta at the base, and generally a sin- 

 gle slight notch near the middle of each lateral margin. 



The lateral surfaces are generally transversely striated, and have 

 a longitudinal pellucid line running down the centre. 



* Lateral view constricted beneath the apex so as to appear urn-shaped. 



1 . G. geminatum, Ag. Densely tufted ; frustules narrow, cuneate, 

 the puncta at the end wanting or nearly so ; lateral surfaces urn- 

 shaped, striated. Ag. Syst. p. 12; Consp. Diatom, p. 35 ; Grev. 

 Crypt. Fl. t. 244. f. 2 ; Kutz. Synop. Diatom, in Linnsea 1833, 

 p. 569 ; Harv. Br. Alg. p. 207. Gomph. ampullaceum, Grev. in 

 Hook. Br. Fl. p. 410. 

 On rocks in subalpine streams. Scotland, Dr. Greville. North 



of Ireland, Mr. D. Moore ; near Bandon, county Cork, Mr. Allman ; 



on dripping rocks near a waterfall at Carnlough, co. Antrim, Mr. W. 



Thompson. Rapid streams at base of Cader Idris, and in the river 



above Dolgelley, N. Wales. 



This plant forms large cushion-like tufts on the rocks in rapid 

 streams. It is not in the least mucous, but is of a spongy tex- 

 ture, being composed of the densely interwoven filaments : in a 

 young state its colour is brownish from the frustules covering the 

 surface, but as these fall off" it becomes whitish, and indeed not 

 unlike a tuft of wool. Filaments repeatedly dichotomous ; frus- 

 tules much larger than in any of the following species, simple or 

 binate, linear-cuneate, without puncta at the upper end ; portions 

 of the lateral surfaces are frequently visible along the sides. La- 

 teral surfaces urn- shaped, broader than the front, with numerous 

 transverse dotted striae. There is a central longitudinal pellucid 

 line slightly dilated at each end, with a larger dilatation in the 

 centre ; the latter at first sight may be taken for a perforation. 



