BRITISH DIATOMACE.E. 35 



striatula, Eng. Bot. 1928. Diatoma striatulum, Grev. B. F. p. 405. 

 Striatella arcuata, Ag. Consp. p. 61. Ehr. Inf. xx. 6. Harv. Man. 

 p. 199. Ralfs, Ann. vol. 11. pi. ix. 6. Prit. Anim. iv. 203, 204. 



Marine. Hastings, Oct. 1851. Cuckmere, Sussex, April 1852. Poole 

 Bay, Sept. 1854, W. Sm. Penzance, Mr. Ralfs. Coast of Sussex, Mr. Jenner. 

 Frith of Clyde, Feb. 1854, Br. Arnott. 



Plate XXXVIII. 305/ 305 a, & 305f. 



2. Rhabdonema Adriaticum, Kiltz. Filament arcuate, substipitate; 

 annuli from 15 to 60, 8 in *00l" ; septa perforate, alternate, 

 gradually decreasing in length as the annuli recede from the valves ; 

 valve linear-elliptical ; striae not extending to the extremities of 

 valve, 24 in -001". Width of filament -001 1" to -0054". Breadth 

 of valve -0004" to '0006". v.v. 



Kiitz. Bacill. xviii. 7. 



Marine. Poole Bay, Nov. 1849, Sept. 1854, and Cork Harbour, Oct, 1855, 

 W. Sm. Belfast Bay near Carrickfergus, July 1854, Mr.Hennedy. Frith of 

 Clyde, Feb. 1854, Dr. Arnott. Ilfracombe, Miss Hodgson. (New York and 

 Mauritius ; communicated by Dr. Arnott. Mediterranean Sea near Marseilles, 

 May 1854, W. Sm. Near Jaffa, Mr. Johnson.) 



The above species is much less frequent than the former on the British 

 coast, and I was unable at first to satisfy myself of its distinctness. The 

 specimens which I collected in the Mediterranean, as well as those supplied 

 by Dr. Arnott from New York, have enabled me to examine the form more 

 thoroughly, and I am now satisfied that it cannot be regarded as a mere 

 variety. The characters I have given are constant in the smallest as well as 

 the largest filaments, and the relative length and alternate arrangement of 

 the perforate septa are so obvious in prepared specimens, that there can be no 

 difficulty in the determination of the species. 



With the present, which occurs very sparingly in the gathering from Mau- 

 ritius, there is another magnificent species with filaments occasionally reaching 

 •0086" in width, and with alternate and cribrose septa ; the latter give a very 

 brilliant appearance to the aspect of the frustule. For this species I propose 

 the name of R. mirificum. I have the same in a gathering from Ceylon, sup- 

 plied by Dr. Kelaart. 



All the species of the present genus occasionally suffer a partial displace- 

 ment of their frustules, which afterwards continue to cohere by their angles ; 

 but this partial rupture is evidently due to external violence, and never occurs 

 with that regularity which gives to Diatoma, Tabellaria and others the 

 notable zigzag which characterizes their filaments. 



Plate XXXVIII. 305 b. & 305 a' & b'. 



3. Rhabdonema minutum, Kiltz. Filament direct or slightly flexu- 



d 2 



