FRAGILARIA. 413 



margins ; in end view curved with slight constrictions at 



each extremity, striated. 



/3 undulata Ralfs. Convex margin of end view of the 

 frustule with two indentations ; centre of concave margin 

 prominent. 

 Ag. Syst. p. 7. ; Consp. Diat. p. 62. ; Grev. in Br. Fl. 



vol. ii. p. 403. ; Harv. Br. Alg. p. 197. Conf. pectinalis 



Dillw., t. 4. ; Eng. Bot. 1. 1611. ; Jurg, Dec. 18. No. 12. 



Fragilaria grandis Ehr., Die Infus. p. 203. 1. 15. fig. 11.; 



Pritch. Infus. p. 220. $ undulata, fig. 171. ; Kalfs, in 



Annals, vol. xii. pi. 2. fig. 3. F. pectinalis Ehr., pi. xvi. 



fig. 1 a ; Jenner, loc. cit. p. 202. 



Hob. Everywhere common in pools, ditches, &c. /3 un- 

 dulata. Drws Ardudwy, near Barmouth : Rev. T. Sal- 

 wey. Scrubbs, Colebrook Park : Mr. Jenner. 



The filaments of this species and others of the genus vary 

 exceedingly in size, or rather in diameter, on account of the 

 cells of the species of the genus developing themselves in 

 breadth as well as length, and this developement occasions 

 the greatest variations in the width of the cells, compared to 

 their length : thus sometimes the cells are longer * than broad 

 in the very slender filaments ; at others, as in the longest 

 threads, they are six or seven times broader than long, but 

 usually their breadth exceeds their length only by three or 

 four times. 



On the anterior surfaces of the frustules at either extremity, 

 but not extending entirely across them, a delicate row of striae 

 may be detected : these add greatly to the beauty and interest 

 of the plant, and do not appear hitherto to have been described : 

 they are doubtless formed by the continuation of the striae 

 observed in the end view of the frustule. Mr. Jenner has 



* The terms length and breadth, when employed in this work in the 

 description of the Diatomacece, are used in the same sense in which they 

 are employed in the definitions of the Conferva, the breadth of the frustule 

 corresponding to the diameter of the filament, and the length to its longi- 

 tude. In the DiatomacecB the breadth of the frustule usually exceeds the 

 length. 



