lOS Mr. J. Ralfs on the Diatomacese. 



interior of the frustule except the corners, where the puncta at 

 the ends are situated ; it is filled with a yellowish granular mass, 

 mixed with numerous colourless vesicles (PI. II. fig. 3. h). 



The lateral surfaces are very characteristic, and will distinguish 

 this from all the following species. One margin is flat, or slightly 

 concave ; the other is convex, and slopes off rather abruptly near 

 the ends, where also it is slightly constricted. Besides these 

 constrictions, indications of two others may often be observed on 

 the convex margin ; but in a specimen sent me by Mr. Salwey 

 these are so strongly marked, that if I had not seen intermediate 

 forms, I should have supposed it to belong to a distinct species. 

 I have made it var. /3. undulata, which perfectly agrees with the 

 usual state of the species, except in the lateral view, the convex 

 margin of which has two indentations, giving it an undulated 

 appearance; the other margin is flat, with a projection in the 

 centre. In both forms the lateral surfaces are marked with close 

 transverse striae. 



Mr. Dalrymple, whose experience and knowledge of these tribes 

 gives great weight to his opinion, informs me that this species is 

 unquestionably the Fragil. grandis of Ehrenberg. It agrees very 

 well in most respects with Ehrenberg^s description and figures 

 except as to the lateral surfaces, which he has described and 

 figured as lanceolate with obtuse ends, a form differing widely 

 from our plant. Mr. Dalrymple, however, observes, that Ehren- 

 berg has one figure striated and another not so, although equally 

 large, and hence he supposes that the junction surfaces may have 

 been figured from a different species, which is represented in the 

 non-striated figure. But whether this plant be Ehrenberg^s 

 Fragilaria grandis or not, I have no doubt that he is in error 

 regarding the species which he has named Fragilaria pectinalis. 

 His mistake was pointed out to me by Mr. Borrer, who at the 

 same time sent me a specimen of the Conferva pectinalis of ^ En- 

 glish Botany,' which is the present plant. I am also enabled, 

 through the kindness of Mr. Berkeley, to refer Jm'gens's plant 

 also to this species. 



Plate II. fig. 3. a, various states of F. pectinalis', b, frustules deprived 

 of their colouring matter; c, lateral view ; d, lateral view of variety /3. 



2. F. rhabdosoma, Eh. Frustules narrow; puncta wanting or indi- 

 stinct; lateral surfaces narrow-lanceolate, without striae. Eh. Die 

 Infus. p. 204. t. 15. f. 12; Pritch. Infus. p. 221. f. 173. Frag, 

 pectinalis, Kutz. in Linnsea 1833, p. 73 } Frag, tenuis, Ag. Consp. 

 Diatom, p. 63 ? 



In freshwater pools, &c. ; common, but generally in small quanti- 

 ties, and mixed with other Diatomacea. Sussex, Mr. Borrer ; Tun- 

 bridge Wells, Mr. Jenner ; Cheshunt, Mr. Hassall ; Barmouth, Rev. 

 T. Salwey ; Portmore Lough, Ireland, Mr. D. Moore ; Stevenston, 

 Ayrshire, Rev. D. Landsborough. Dolgelley, Ilfracombe, Penzance. 



