Mr. J. Ralfs on the British species of Gomphonema. 461 



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

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

 the upper end. 



I have drawn 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 numerous most valuable specimens of 

 Algse 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. 

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

 turn, 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 Vaucherim 

 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. Gomphonema geminatum. 



2. G. pohlioeforme, 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. 18. f. 1 ; Pritch. Infus. p. 227. f. 187- 

 190. Gomph. pohli(Bformey Kutz. Alg. aq. dulc. No. 25 ! (young.) 

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

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

 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. Salwey, Ilfra- 



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



This is a very minute species, which to the naked eye seems 

 only a brownish 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 sur- 

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

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

 and the lateral view is but little constricted, but when mature 

 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 perfect 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. 



