Mr. J. Ralls on the British species of Gomphoiiema. 461 



which it undoubtedly is not ; from this central punctum the strise 

 are somewhat radiant : the pellucid line does not quite extend to 

 the upper end. 



I have di-awii up the description of this plant from a specimen 

 of Dr. Greville's given to me by Mr. Harvey, to whom I feel much 

 indebted for his kind communications, as well as for the liberality 

 with which he has added nimierous most valuable specimens of 

 Algae to my collection. 



I regret that I have been unable to obtain an authentic speci- 

 men of Dr. Greville's Gomphonema ampullaceum, but from the de- 

 scription I have little hesitation in uniting it with this species. 

 Blr. Moore, who considers it distinct from Gomphonema gemina- 

 tum, has sent me specimens under both names, but I cannot dis- 

 cover any essential difference between them. Mr. Moore makes 

 the following observations on Gomph. ampullaceum : — " This, be- 

 sides growing in much larger tufts than G. geminatum, is of a 

 much greener colour when recent, resembling some Vaucheria 

 going into decay. The frustules are much constricted below the 

 apex, and have generally a more pellucid line running down the 

 centre." 



Plate XVIII. fig. 3. Gomplionema geminatum. 



2. G. pohliceforirie, Kutz. Very minute ; frustules cuneate, with two 



evident puncta at the upper end ; lateral surfaces urn-shaped, 



striated. Kutz. Synop. Diatom, p. 570. f. 50. Gomph. truncatum. 



Eh. Die Infus. p. 216. t. IS. f. 1 ; Pritch. Infus. p. 227. f. 187- 



190. Gomph. pohlieeforme, Kutz. Alg. aq. dulc. No. 25 ! (young.) 



Gomph. subramosum, Kutz. Synop. Diatom, p. 570. fig. 44. Alg. 



aq. dulc. No. 152 ! (mixed with Gomph. minutuni). 



On aquatic plants in freshwater pools and ditches : autumn and 



spring. Near Henfield, Mr. Borrer ; Tunbridge Wells, Mr. Jenner ; 



Cheshunt, Mr. A. H. Hassall \ Oswestry, Rev. T. Salivey; Ilfra- 



combe, Swansea, Dolgelley and Bangor, N. Wales. 



This is a very minute species, AA^hich to the naked eye seems 

 only a bro\^^lisll discoloration of the plant on which it grows. 

 At first the frustules are frequently quite sessile, but ultimately 

 they become elevated on a simple or slightly branched stipes, when 

 they are sometimes clustered four or six together ; they are cu- 

 neate with two evident puncta at the upper end. The lateral sm*- 

 faces are constricted below the apex so as to be m-n-shaped, and 

 are attenuated at the base. Wlien young the frustules are short 

 and the lateral view is but little constricted, but when matm-e 

 they are longer and the lateral view becomes more completely 

 urn-shaped. In the former state it is figured by Kutzing and 

 Ehrenberg ; in the more jierfect form it resembles Ehrenberg^s 

 figm'e of Gomphonema capitatum, nor is there any character in his 

 description to distinguish the latter from this species. 



