34 BRITISH DIATOMACEjE. 



( = Rhabdonema minutum, Kiitz.), they are shortest near the angles, and 

 gradually longer as they approach the middle. In the latter the 

 outlines of two plates are frequently seen at one time when viewed 

 laterally, in consequence of the unequal size of the plates " (see 

 Plate XXXVIII. 306 a & a", Plate XXXIX. 307 h). 



We shall find that the septa, here so accurately described, are not 

 invariably projected equally from all portions of the annuli, but that 

 an annulus will project a septum from one side only, and the next 

 annulus a septum from the side opposite its neighbour ; in such cases 

 the septa appear alternately on opposite margins of the filament, as 

 may be seen in Rhabdonema minutum, Striatella unipunctata and Ta- 

 bellaria flocculosa, while opposite septa, where the development takes 

 place symmetrically around the interior of each annulus, may be noted 

 in Rhabdonema arcuatum and Grammatophora marina. The analyses 

 given in Plate XXXVIII. 305f, Plate XLII.314-J-, Plate XLIII.316f 

 & 3 1 7t> will enable the student to understand the above description, 

 and will very clearly exhibit to the eye the singular structure I have 

 attempted to describe. 



It will be seen that the annuli in the present genus are furnished 

 towards the circumference with a single series of oblong cellules ; 

 these give a striated appearance to the F. V. of the frustule, and the 

 number of these striae will accurately represent the number of annuli. 

 It is hardly necessary to say, that the peculiar structure of the frus- 

 tule separates the present genus from any previously described, the 

 form of its filament from Tabellaria and Tetracyclics, the coherence 

 of its frustules from Grammatophora, and its cellulate annuli from all 

 these and Striatella. I have detected conjugation and the formation 

 of sporangia in one of our native species, viz. R. arcuatum. R. minu- 

 tum, though occurring in great abundance in various localities and 

 seasons, on the coast of Sussex, has never presented this phasnomenon 

 to my observation. 



1. Rhabdonema arcuatum, Kiitz. Filament slightly arcuate, sub- 

 stipitate ; annuli from 9 to 30, 6 in *001" ; septa entire, opposite, 

 slightly and variably unequal in length ; valve linear-elliptical ; 

 striae not extending to the extremities of valve, 16 in # 00l". Width 

 of filament -0011" to -0041". Breadth of valve -0006". v.v. 



Kiitz. Bacill. xviii. 6. Biatoma arcuatum, Lyng. Tent. lxii. B. Conferva 



