Mr. J. Ralfs on the Diatomaceee. 3^71 



are smaller than those of the sides. On each side of this central 

 portion are seen the lateral surfaces, with their reticulations and 

 striae arranged transversely. As these lateral sui'faces are turgid 

 and compressed, they appear at first sight to form part of the 

 front of the frustule. This may be observed in some slight degree 

 in Diatoma vulgar e^ but not to the same extent as in this genus. 



That it is in fact a portion of each lateral surface which is thus 

 seen in front will be found on a careful examination of the exterior ; 

 but it is still more evident on a view of the interior of a fractured 

 frustule, when the junctions of the sides with the central portion, 

 marked externally by mere lines of separation, form internally 

 projecting plates or rims. 



The mode of growth in this genus is very curious. In Diatoma, 

 as well as in most of the DiatomacecBj the plant increases by a 

 division of the frustules ; but in this genus, and also in Biddulphia 

 and AmphitetraSy two new frustules are formed within the old 

 one, and, as they enlarge, rupture it, when it falls off. In these 

 the front portion is at first very narrow and merely a broad line, 

 but it increases greatly in breadth until the new frustules are 

 fully formed. 



1. /. enervis, Eh. Frustules generally much broader than long, tho 

 sides with large reticulations, not striated, or else with the striae 

 between each row of cells. Isthmia enervis, Eh. Die Infus. p. 209. 

 t. 16. f. 6 ; Pritch. Infus. p. 223. f. 183. Isthmia ohliquata. Hook. 

 Br. Fl. p. 405 ; Harv. Br. Alg. p. 201. Isthmia ohliquata, /3 te- 

 nuior, Ag. Consp. Diatom, p. 55. Biddulphia ohliquata, Gray, Arr. 

 p. 294. Conf, ohliquata, E. Bot. t. 1869. 



/3. suhcylindrica. 



Bally water, co. Down, Ireland, Mr. Thompson', Torquay and Ilfra- 

 combe, Devonshire ; Bracelet Bay near Swansea. 

 /3. Torquay, Mrs. Griffiths. 



Ehrenberg considers the ^Eng. Bot.' figure to belong to the 

 next species, but I have no doubt that it is intended for this 

 plant, of which it is a very good representation. The highly mag* 

 nified figures in ' Eng. Bot.' undoubtedly represent it ; those that 

 are less magnified are too broad for the frustules of his /. ohli- 

 quata, whilst under a low power this species appears striated like 

 them. 



The trivial name ohliquata, which was originally applied to this 

 species, should I think have been retained ; but, on account of the 

 celebrity of Ehrenberg's work, I have not thought it expedient to 

 change his name. 



Under this species, as I am informed by Mr. Dalrymple, 

 Ehrenberg gives no synonyms and only one habitat, — Gbthen- 



