52 BRITISH DIATOMACE^5. 



reduce it to some rule by assigning to /. nervosa all species which 

 have been described as " striated ;" though still in doubt whether the 

 arrangement of the cellules may not in some cases, under imperfect 

 instruments, have been regarded as " striation." 



1. Isthmia nervosa, Kiitz. Valves inflected towards the suture, 

 transversely lined by costae which anastomose and disappear to- 

 wards the centre of valve ; cellules of connecting-membrane much 

 enlarged near the suture. Length of valve •0024" to - 0082". v.s. 



Kiitz. Bacill. xix. 5. Isthmia obliquata, Ag. Consp. p. 55. Ehr. Inf. xvi. 5. 

 Ralfs, Ann. vol. 12. viii. 2. Harv. Man. p. 200. Diatoma obliquatum, 

 Lyng. Tent. lxii. c. 2. 



Marine. Carnlough Bay, North of Ireland, Mr. D. Moore ; communicated 

 by Mr. Jenner. "Near Glenarne," 1847, Mrs. Lyon; communicated by 

 Dr. Landsborough. Balta Sound, Mr. Edmonstone, communicated by Mrs. 

 Griffiths. 



Plate XLVII. 



2. Isthmia enervis, Ehr. Valves direct, not inflected ; cellules of 

 connecting-membrane slightly enlarged at the suture. Length of 

 valve -0024" to -0082". v.v. 



Ehr. Inf. xvi. C. Ralfs, Ann. vol. 12. viii. 1. Kiitz. Bacill. xix. 4. Prit. 

 Anim. iv. 183. Isthmia obliquata, /3. tenitior, Ag. Consp. p. 55. Con- 

 ferva obliquata, Eng. Bot. tab. 1869. 



Marine. Torbay, July 1846 ; Cork Harbour, Aug. 1855, W. Sm. Tor- 

 bay, Mrs. Griffiths. Ilfracombe, Mr. Ralfs. Maiden Rocks, near Larne, 

 Br. Dickie. Haverfordwest, July 1854, Mr. Okeden. 



Plate XLVIII. 



Genus 48. PODOSIRA, Ehr. 



Filament cylindrical, of few frustules, attached ; frustules spherical or 

 cylindrical, geminate by the persistence of the connecting-mem- 

 brane as a siliceous cingulum; valves hemispherical, minutely 

 punctate or cellulate ; silex absent from the apex of the valve. 



Having employed the word " annulus" in another connexion, I 

 adopt the term " cingulum " in the present and following genus to 

 denote the subpersistent condition of the connecting-membrane, 



