272 Mr. J. Ralfs on the Diatomacese. 



berg. I believe, however, that most of the habitats assigned by 

 him to /. obliquata will prove to belong to this species. 



The frustules are generally from two to three times broader 

 than long, but sometimes they are nearly equal in length and 

 breadth. This is more particularly the case in the newly-formed 

 frustules. The front or " broad transverse band" of British works 

 generally occupies from one-quarter to one-third of the frustule, 

 and its reticulations are roundish and very much smaller than 

 those of the sides. This portion is separated from each lateral 

 portion by a rather broad line formed by the junction and inflec- 

 tion of the two portions. The cells bordering on this line are 

 not remarkably larger than the other cells in this band. 



The cells in the lateral surfaces are of a square form and are 

 arranged in regular transverse lines separated from each other by 

 pellucid lines, which viewed from within form ridges and are as 

 truly strise as those in the next species, and under a low power 

 the frustule appears with numerous close strise as seen in the 

 figure in ' Eng.Bot./ which I have already referred to this species. 



Mrs. Griffiths has sent me two forms from Torquay ; one like 

 that described above, the other with frustules much smaller and 

 broader, and so turgid as to be nearly cylindrical. Of this latter 

 I have seen but a very small portion, and am uncertain whether 

 it is a young state of this species or wholly distinct. 



Plate VIII. fig. 1. a, Isthmia enervis', b, variety /3. 



2. /. obliquata, Ag. (Eh.) Frustules rather broader than long, the 

 front with a series of large cells along each separating line, the 

 sides with but few and rather distant strise. Isthmia obliquata. 

 Eh. Die Infus. p. 209. t. 16. fig. 5; Pritch. Infus. p. 223 ; Ag. 

 Consp. Diatom, p. 55?; Kutz. Synop. Diatom, in Linnsea 1833, 

 p. 579. 1. 16. fig. 69? 

 Carnlough Bay, co. Antrim, Mr. Moore, Dr. Drummond, and Mr. 



Thompson. Mr. Moore has also gathered it in the co. of Wicklow. 



This and the preceding species have been so confused, that I 

 quote the above synonyms with some doubt. Ehrenberg consi- 

 ders Kutzing^s figure to belong to this species ; and as it exhibits 

 series of larger cells bordering the separating lines, I have quoted 

 it under this plant, but I am by no means certain whether it is 

 intended for it. 



On examining recent plants of Isthmia gathered at Ilfracombe 

 in October last, it appeared to me that all my specimens be- 

 longed to /. enervis of Ehrenberg and not to his /. obliquata, and 

 Mr. Dalrymple having compared one of them with Ehrenberg's 

 figure, ascertained that my supposition was correct. About the 

 same time a very valuable packet of Irish Diatomacece was sent to 

 me by Mr. Moore, amongst which I was much pleased to find 



