6 BRITISH DIATOMACE.E. 



to remark that the width of the filament equals the length of the 

 frustule or valve measured along the suture or junction line, and that 

 the breadth of the valve denotes the thickness of the filament. 



1 . Meridion circulare, Ag. Frustules on F. V. cuneate, truncate ; 

 V. cuneate, rounded at the larger extremity, gradually attenuated 

 towards the other, which is slightly inflated and obtuse ; costae 

 unequally distant, frequently interrupted ; striae obscure, 40 in 

 •001". Length of frustule -0013" to -0028". v.v. 



/3. Frustules with internal cells, v.v. 



Ag. Consp. p. 40. Grev. B. F. p. 409. Harv. Man. p. 205. Ralfs, Ann. 

 vol. xii. pi. xviii. 1. Hass. Alg. xcvi. 1-6. Quek. H. C. xii. 2. Kiitz. 

 Bacill. vii. 16 ; Sp. Alg. p. 10. Raben. Siissw. Diat. Taf. 1. Meridion 

 vernale, Ag. Syst. p. 2. Ehr. Inf. xvi. 2. Prit. Anim. iii. 1 77. Echinella 

 circularis, Grev. S. C. F. pi. 35. Var. (5. Meridion Zinckeni, Kiitz. 

 Bacill. xvi. 8 ; Sp. Alg. p. 10. Raben. Siissw. Diat. Taf. 1. ad specim. 

 quae communicavit am. De Brebisson. 



Fresh water : very frequent. Lulworth, Dorset, May 1849. Near Lewes, 

 April 1850, March 1852, March and Dec. 1853. River Lee near Cork, 

 April 1855,&c, W. Sm. Firle and Barcombe, Sussex, June 1842, Mr. Jenner. 

 Aberdeenshire, Sept. 1847, Dr. Dickie. Var. /3. Hornby, Lancashire, May 

 1852, Mr. Geo. Smith. Colin Glen near Belfast, Dr. Dickie. Kendal, March 

 1854, Miss Hodgson. (River Sorgues, Vaucluse ; Fountain at Nismes ; 

 Royat, and Puy de Dome, elev. 3000 feet, May and June 1854.) 



There are few of our native Diatomacese more interesting to the observer 

 than the present beautiful species, and few which more frequently present 

 themselves to his notice. Its usual habitats are the sources of clear streams, 

 especially in chalk or limestone districts ; but it not unfrequently occurs in 

 open streams or ditches, especially if the water be fresh and pure. It may 

 always be found in its earlier stages of development attached to larger plants 

 by its first-formed frustule by means of a minute gelatinous cushion ; but the 

 attachment is so evanescent that the slightest force seems sufficient to effect 

 a separation, and in its more advanced condition the filaments are invariably 

 detached. 



Professor Kiitzing has removed the next species from the present genus on 

 the ground of the former being attached or stipitate ; but from what I have 

 just said, it will be apparent that there exists no reason for such a separation. 

 In truth, as far as my own observation extends, the facts are apparently the 

 reverse of those upon which he relies, — M. circulare being at first attached, 

 and M. constrictum usually free ; I am however disposed to believe that in 

 both cases an attachment exists, but only of an imperfect and temporary 

 nature, and that in the second species named it is even more slight and 

 evanescent than in the former. It not unfrequently happens, in still water, 

 that the mucus which is produced during the process of self-division accumu- 



