BRITISH DIATOMACE^. 7 



lates around the dividing frustules and forms a considerable mass ; in such 

 cases the growing plant bears a close resemblance to Gomphonema olivaceum 

 in a similar stage, and may be easily mistaken for it. 



In var. ft. we meet with a curious modification in the growth of the frus- 

 tule, which has been regarded by some observers as characteristic of a distinct 

 species. It will be afterwards seen that this view would lead, not unfrequently, 

 to the removal of similar forms from the species to which they clearly belong, 

 and from which the modification in question seems insufhcieut to warrant 

 such a separation. A close examination of such frustules, especially in a 

 living state, has led me to the conclusion that the appearance of a double wall 

 of silex is owing to the formation within the original frustule of a second 

 perfect cell, instead of the usual mode of division by which the original frus- 

 tule is divided into two half-new cells, as described in the Introduction to the 

 present work, Vol. I. p. xxiv. In the present case the central vesicle or 

 cytoblast becomes enlarged without division, and secretes en its extension two 

 new valves, which are pushed outwards until they lie in close approximation 

 with the original valves. This process is not always repeated, the usual mode 

 of self-division again recurs, and two valves are formed in the interior of this 

 new cell according to the normal method ; hence we have the appearance pre- 

 sented in var. ft. of the present and following species, as figured in Plate 

 XXXII. 277 ft. and 278 ft. This unusual method of development is not 

 however sufficiently constant to warrant the separation of such frustules from 

 the species in which it occurs, perhaps hardly sufficient to constitute a variety, 

 as frustules in both the ordinary and abnormal states may be met with in the 

 same gathering, and even in the same filament. 



Plate XXXII. 277. Var. ft. 277 jS. 



2. Meridion constrictum, Ralfs. Valve constricted towards the 

 larger extremity, otherwise like the last. Length of frustule *0006" 

 to -0017". v.v. 



ft. Frustule with internal cells, v.v. 



y. Frustule on S. V. nearly linear. Length -0023" to -0030". v.s. 



Ralfs, Ann. vol. xii. pi. xviii. 2. Hass. Alg. xcvi. 7 & 9. Jenn. F. Tun. 

 p. 206. Raben. Siissw. Diat. Taf. 1. Eumeridion constrictum, Kiitz. 

 Bacill. xxix. 81; Sp. Alg. p. 11. 



Freshwater: frequent in boggy pools. Tunbridge Wells, April 1843, 

 Mr. Jenner. Grassmere, Aug. 1853, W. Sm. Near Lancaster, Feb. 1852, 

 Mr. Johnson. Davey Hulme near Manchester, Prof. Williamson. Near 

 Aberdeen, March 1848, Dr. Dickie. Premnay Peat. (Berlin Fossil Earth, 

 near Falaise, M. De Brebisson.) Var. ft. Grassmere, Aug. 1853. Moanarone, 

 county Cork, April 1855. River Spey, July 1854, Dr. Gregory. (Genolhac 

 in the Cevennes, elev. 3600 feet. Puy du Cliergue, Mont Dore, elev. 5577 feet, 

 W. Sm.) Var. y. Grassmere, Aug. 1853, W. Sm. 



