70 BRITISH DIATOMACE^E. 



2. Colletonema vulgare, Thw. Frond filiform, occasionally divided, 

 gradually acuminate, containing one or two rows of frustules in 

 regular file ; V. elliptical-lanceolate, slightly contracted towards 

 the obtuse extremities; striae 72 in -001". Length of frustule 

 •0018" to -0030". Breadth of V. -0004" to -00055". v.v. 



Thw. Ann. ser. 2. vol. 1. pi. xii. H. 



Fresh water. Near Bristol, Dec. 1847, Mr. Thxoaites. Near Lancaster, 

 April 1852, Mr. Johnson. (Falaise, M. De Brebisson. Cascade de la 

 Verniere, Mont Dore, June 1854, W. Sm.) 



This is hy no means the common species represented by Mr. Thwaites, who 

 says that " it occurs during the spring in almost every ditch and running 

 stream," a mistake which appears to have arisen from confounding it with 

 the next species, which is far more general. To distinguish between the 

 two requires the careful use of the microscope, as the form of each is so 

 nearly alike. The principal difference is to be found in the character of 

 the striae, which are tolerably conspicuous in C. neglectum, and extremely 

 faint and delicate in C. vulgare. Mr. Thwaites describes three varieties of 

 the present species, viz. a. rivulosum, /3. lacustre, y. effusum ; but the di- 

 stinctions between them depend mainly upon the stage and place of growth ; 

 the first and last occur in streams, var. a. with a divided frond, and 

 var. y. forming a gelatinous covering to stones, with no appearance of 

 separate filaments, — while var. /3. is characterized by a simple filament, and 

 is met with in still water, or where there is only a slight current : the frus- 

 tules of all three are described as having " a lanceolate form, suddenly 

 narrowed towards the apices." Not having found the species in more than 

 one locality, I am unable to confirm this account. 



Plate LVI. 351. 



3. Colletonema neglectum, Thw. Filament slightly divided, ob- 

 tuse, containing numerous and closely packed frustules ; V. ellip- 

 tical-lanceolate ; extremities obtuse ; striae 32 in '001". Length of 

 frustule -0015" to -0024". Breadth of valve -00035" to -00045". v.v. 



Thw. Ann. ser. 2. vol. 1 . pi. xii. J. 



Fresh water. Near Bristol, Dec. 1847, Mr. Thwaites. Near Lewes, April 

 1851, Nov., Dec. and Jan. 1853, W.Sm. 



Plate LVI. 352. 



4. Colletonema subcohaerens, Thv). Frond globose, gelatinous, 

 pervaded by files containing from one to five rows of frustules ; 

 V. elliptical-lanceolate, with rounded extremities ; striae 28 in -001". 



