FRAGILARIA. 413 



margins ; in end view curved with slight constrictions at 

 each extremity, striated, 



/3 undulata Kalfs. — Convex margin of end view of the 

 frustule icith two indcjitations ; 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. ; Eug. Bot. t. 1611. ; Jurg, Dec. 18. No. 12. 

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

 Pritch. Infus. p. 220. — /3 vndulata, fig. 171. ; Ralfs, in 

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

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



Hah. 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 surfiices of the frustules at either extremity, 

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

 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 stride 

 observed in the end ^iew of the frustule. ^Ir. Jenner has 



* The terms lengtli and hrcudth, M-hon employed in this work in the 

 description of the Diatomaccce, are nsed in the same sense in which they 

 are employed in the definitions of the Confervce, the breadth of the frnstule 

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

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

 length. 



