38 BRITISH DIATOMACEiE. 



gatherings from which Ehrenberg derived his specimens. I feel 

 assured that all the species of Biblarium are filamentous in a living 

 state, and that the greater number of them are casual varieties of 

 Tetracyclus lacustris. 



1. Tetracyclus lacustris, Ralfs. Extremities of valve rounded or 

 subacute; inflations rounded; annuli 12 to 40, 6 in *00l" ; septa 

 alternate, equal in length. Width of filament -0008" to -0022". 

 Breadth of valve at the inflation -001". Costae 7 to 12, 6 in 

 •001". v.v. 



Ralfs, Ann. vol. 12. pi. iv. 2. 1 843. Kiitz. Bacill. xxix. 70. Prit. Anim. xiv. 

 24, 25. Raben. Siissw. Diat. pi. ix. Biblarium Stella, B. Glans, and B. 

 speciosum, Ehr. in Kiitz. Sp. Alg. p. 117, et Microgeologie, passim. 



Fresh water. Dolgelly, Aug. 1843, Mr. Ralfs. Tonabrick Mountain, Co. 

 Galway, July 1853, W. Sm. River Spey, July 1854, Dr. Gregory. Dolgelly 

 Earth. (Lapland.) 



Plate XXXIX. 308 : margin of a frustule, 308 x. 



2. Tetracyclus emarginatus, TV. Sm. Valves constricted towards 

 the extremities, which are rounded and subapiculate ; inflations 

 deeply notched or emarginate ; otherwise like the last species. 



Biblarium emarginatum, Ehr. in Kiitz. Sp. Alg. p. 118, et Microgeologie, 

 passim. 



Fresh water. Mountain streams and cascades, Gap of Dunloe, Killarney, 

 July 1855, W. Sm. River Spey, Dr. Gregory. Killiecrankie, Dr. Greville. 



In the Killarney gathering this form occurs abundantly, and unmixed with 

 the last ; but, in the River Spey, it is sparingly present along with undoubted 

 frustules of Tetracyclus lacustris. It is possibly only a variety of the latter, 

 though its aspect is so different that the observer cannot fail to detect its pre- 

 sence, and to regard it as distinct. I regret that it was noticed too late to be 

 figured in the present work. 



Genus 42. DIATOMA, Dec. 



Frustules oblong, adhering by their opposite, and alternate, or more 

 rarely by their adjacent angles into a zigzag chain ; valves elliptical 

 or linear. 



The position of the present genus, with its ordinary cell, in a sub- 

 tribe whose genera have annulate frustules, is somewhat anomalous; 



