18 BRITISH DIATOMACE^E. 



A very curious form, frequent in clear springs and mountain lakes, but 

 overlooked from its minute size, and the close resemblance of the S.V. of its 

 valve to small specimens of Tabellaria flocculosa ; this aspect of the frustule 

 differs, however, from the same in that species in its acuminate extremities 

 and broader and more rounded inflation, and its mode of increase places it 

 widely apart from the genus Tabellaria. Var. ft. is a singular modification 

 of growth, which frequently occurs intermixed with the other form. Whether 

 the synonyms from the 'Microgeologie' belong to either or both these forms, 

 I have no certain means of determining. 



Plate XXXIV. 291. Var. ft. 291 ft. 



6. Odontidium? Harrisonii, n. sp. Filament attached, of few 

 frustules, which frequently adhere by their angles only; valve 

 cruciform, angles rounded; costae not reaching centre of valve, 

 distinct, 13 in -001". Length of frustule -0006" to -0018". v.v. 



ft. A smaller form with more acute angles. Length of frustule - 0005" to 

 •0009". v.s. 



Var. ft. Roper, Journ. Micr. Sci. vol. ii. pi. vi. 6. 



Fresh water. Near Hull, April 1852, Aug. 1849 and Jan. 1854, Mr. 

 R. Harrison. Var. /3. Aberdeenshire, Dr. Dickie. Burnham, Norfolk, Jan. 

 1854, Mr. Brookes. River Thames, Jan. 1854, Mr. Roper. Killiecrankie, 

 August 1854, Dr. Greville. Lough Leven, Dr. Gregory. Lough Derg, Co. 

 Clare, July 1855, W. Sm. fyc. 



The larger form has alone occurred in the locality where it was first dis- 

 covered by Mr. Harrison; the smaller appears widely distributed, as it is 

 sparingly present in almost every alpine or subalpine gathering with which I 

 am acquainted. Though I have not seen a living specimen of the latter, yet 

 it so closely resembles the larger form in everything but the greater acute- 

 ness of its angles, that I have not hesitated to regard it as a variety. 



The position of O. Harrisonii, in a systematic arrangement, is a matter of 

 doubt. The zigzag connexion of its frustules in the Hull specimens seems 

 to point to Diatoma as its nearest ally, but the interrupted costse remove it 

 from that genus. Its structure and mode of growth are essentially different 

 from Tetracyclus, to which the cruciform outline of its valve otherwise 

 seems to approximate it. I should perhaps have placed it in Staurosira 

 had I known the characters Ehrenberg assigns to that genus, but having 

 only the figures of the 'Microgeologie' to guide me, I cannot be certain 

 that Ehrenberg's definition would include my species. His outline of 

 Staurosira pinnata, Microg. tab. v. ii. 24, is very like my Var. ft. ; but I 

 do not see any figure in that work that can be regarded as representing 

 the larger form of the present species. On the whole, being averse to 

 constitute a new genus for the reception of a single species, I am disposed 



