Mr. J. Ralfs on the Diatomaceae. 45 1 



(young), Kutz. in Linnaea 1833, p. 580. f. 60 ? D. tenue, |3. m- 

 termedium, Kutz. p. 580. f. 61 ? D. flocculosum, Ag. Consp. Crit. 

 Diatom, p. 53. exc. syn. Dillw. Bacillaria vulgaris, Eh. p. 197. 

 pi. 15. f. 2 ; Pritchard's Hist, of Infusoria, p. 217. f. 168. 



Plate VIII. fig. 8. a, front view ; b, side view. 



Pools and streams. King's Cliff, Northamptonshire, Rev. M. J. 

 Berkeley, Henfield, Sussex, i&r. Borrer; Shoreham, Kent, and several 

 stations about Lewes and Tunbridge Wells, Mr. Jenner ; Shrews- 

 bury, Mr. Leighton ; Oswestry, Shropshire, Rev. T. Salwey ; Ches- 

 hunt, Herts, Mr. A. H. Hassall; Ireland, Mr. D. Moore. 



This species is distinguished from D. tenue by the greater 

 breadth of the frustules, by their convexity, and by the lateral striae 

 being more apparent along the margin. The frustules have two 

 puncta at each end, and the margin between them is slightly 

 rounded. These puncta are wanting, or much less distinct in the 

 other species. 



As the frustules are thicker than in the other species, it is more 

 easy to obtain a view of the lateral surfaces. These are also con- 

 vex, so that the strise, even before the separation of the frustules, 

 appear to occupy part of the front surface. 



It is brownish when recent and turns green in drying. 



2. D. tenue, Ag. Frustules plane, four times or more longer than 

 broad ; sides mostly parallel. D. tenue, Ag. Consp. Diatom, p. 52. 

 D. tenue, d. normale, Kutz. in Linnaea 1833, p. 580. t. 17. f. 63. D. 

 elongatum, Berk. Brit. Alg. t. 6 ; Hook. Br. Fl. p. 406 ; Harv. Brit. 

 Alg. p. 202. D. sulphurascens, Ag. Consp. Diatom, p. 53 ? Conf. 

 flocculosa, Dillw. Br. Conf. t. 28. f . C ? Bacillaria pectinalis, Eh. 

 Infusoria, p. 198. pi. 15. f. 4 ; Pritchard's Infusoria, p. 218. 



Plate IX. fig. 1. a, front view ; b, side view. 



Pools and streams. Near Tunbridge Wells, Mr. Jenner; Shrews- 

 bury, Mr. Leighton ; Cheshunt, Mr. A. H. Hassall ; Bangor and 

 Dolgelley, N. Wales ; Penzance : Aberdeen, Dr. Dickie ; Steven- 

 ston, Ayrshire, Rev. D. Landsborough. 



This is a very protean species. Sometimes the frustules are 

 thrown quite back until the ends touch, so as apparently to form 

 a filament, the width of which is much less than the length of the 

 frustules. In this state it forms the D. elongatum of Berkeley's 

 ' Brit. Algae/ and the D. tenue of Agardlr's ' Consp. Crit. Diatom/ 

 Indeed Agardh in his ' Conspectus • forms a section of Diatoma 

 upon this very circumstance of the frustules being longer than the 

 breadth of the filament. 



Sometimes the frustules are thrown back in pairs, and then ap- 

 pear like filaments united longitudinally ; but with a little atten- 

 tion the observer will understand this arrangement, and he will 

 derive assistance in tracing it from the connected angles having 



