Mr. J. Ralfs on the Diatomacese. 107 



frustules are mostly longer than broad, with two puncta at each 

 end, and without strige on the central portion. 



The young frustules are often nearly equal in length and 

 breadth, but as they afterwards increase in length a good specific 

 distinction cannot be founded on these characters, especially as 

 the filaments are attenuated. 



I have frequently examined the puncta so generally present at 

 the ends of the frustules in this and other genera of the Cym- 

 belleae, but I have never found any reason to beheve that they are 

 openings, as many naturalists have supposed. Mr. Borrer in- 

 forms me that in Fragilaria pectinalis they are the terminations 

 of slight grooves. 



At present I shall describe only the freshwater species, and 

 defer the consideration of the marine species, diatomoides and 

 auretty until I have more fully examined them. 



1. F. pectinalis, Lyngb. Frustules broad, with two evident puncta 

 at each end ; lateral surfaces striated, curved, constricted on one 

 side near the end. Ag. Syst. p. 7 ; Consp. Diat. p. 62 ; Grev. in 

 Br. Fl. vol. ii. p. 403 ; Harv. Br. Alg. p. 197. Conferva pectinalis, 

 Dill. t. 24; E. Bot. t. 1611!; Jurg. Dec. 18. no. 12!. Frag, 

 grandis. Eh. Die Infus. p. 203. t. 15. fig. 11 ; Pritch. Infus. p.220. 

 /8. undulata, fig. 171. 



In freshwater pools and ditches ; often abundant. Hurst, Sussex, 

 Mr. Borrer ; Lewes and Txinhridge Wells, Mr. Jenner; Cheshunt, 

 Herts, Mr. Hassall; Barmouth, N. Wales, the Rev. T. Salwey ; Aber- 

 deen, Dr. Dickie; Ballantrae, Ayrshire, Mr. Thompson ; Stevenston, 

 Rev. D. Landsborough ; Ireland, Mr. D. Moore. Dolgelley, Penzance, 

 Ilfracombe, &c. 



/3. Drws Ardudwy near Barmouth, the Rev. T. Salwey. 



This plant is brown when recent, but when dried becomes of 

 a pale grayish green colour with a glass-like lustre. 



The frustules are larger in this than in any of the following 

 species. They are broad but vary in length, being from three to 

 five times longer than broad, and generally have the strise clearly 

 visible at their junction. The endochrome is commonly contracted 

 into two irregular hues, which are not unfrequently united in the 

 centre ; but often it is in four patches, apparently from the divi- 

 sion of these lines. The frustules sometimes have a central pel- 

 lucid spot, which does not appear to be connected with the endo- 

 chrome. 



I have several times met with a remarkable state of this species, 

 and have also received it from Mr. Jenner. Within the frustule 

 there is apparently another siliceous frustule, the lateral margins 

 of which are rounded, having striae like the outer frustule. In the 

 longer frustules it is nearly elliptic, but in the shorter ones ap- 

 pears as if truncated at the ends, and in both it occupies the whole 



