14 BRITISH DIATCMACEjE. 



6. Himantidium gracile, Ehr. Valve arcuate or direct, linear; ex- 

 tremities rounded, somewhat inflated or slightly recurved ; striae 

 27 in -001". Length of frustule -0035" to -0065". Breadth of 

 valve -0003". v.v. 



Kiitz. Bacill. xxix. 40. ad specim. quae dedit auc. De Brebisson. 



Fresh water. Killicrankie, Mr. P. Grant. Dolgelly Earth. Lough Mourne, 

 Lough Island Reavey, and Mull Deposits. (Fountain at Nismes ; and Pic 

 du Capucin, Auvergne, elev. 4567 feet, June 1854, W. Sin.) 



Plate XXXIII. 285. 



7. Himantidium majus, W. Sm. V. arcuate, dorsum elevated, 

 extremities inflated and rounded ; striae 27 in -001". Length of 

 F. -0055" to -0075". Breadth of valve -0006". v.s. 



(8. with two dorsal ridges. 



/3. Himantidium bidens, Greg. M. J. vol. 2. iv. 21. 



Freshwater. Gap of Dunloe, Killarney, July 1855, W. Sm. Forfarshire, 

 Dr. Dickie. Braemar, Dr. Balfour, Aug. 1854. Var. j3. Mull Deposit, 

 Dolgelly Earth, &c. 



Differs little from H. gracile, save in its greater size and elevated dorsum, 

 and is probably a sporangial form of it, or some other species. 



Plate XXXIII. 286. Var. /3. Supp. Plate LX. 286 (3. 



Sect. III. Filaments tenacious : marine. 



8. Himantidium? Williamsonii, n. sp. Frustules direct; F. V. 

 with central, terminal, and two intermediate inflations; central 

 inflation with an acute angle, the others rounded ; valves striated ; 

 striae 20 in '001". 



Marine. Dredged off the Island of Skye by Mr. G. Barlee ; communicated 

 by Prof. Williamson. 



I have not been able, in the few specimens I possess of this interesting 

 form, to obtain a S. V. of the frustules, and am in consequence in some doubt, 

 whether it might not be referred to another genus, probably to Biddulphia, 

 to which the inflations on the surface of the valve and its marine habitat 

 seem to ally it. The general appearance and tenacity of the filament are, 

 however, those of the present genus, with which it must remain associated 

 until a further examination confirms its present position or necessitates its 

 removal. 



Plate XXXIII. 287. 



