30 BRITISH DIATOMACEiE. 



from Falaise, I find the former with a long, and the latter with an abbre- 

 viated stipes. Nor do I think the form of the V., though varying from 

 lanceolate with acute ends, to linear-lanceolate with obtuse and somewhat 

 capitate extremities, any certain criterion, as I find these several forms 

 intermixed in the same gathering. On the whole, I am disposed to believe 

 that we have in Britain, as far as has hitherto been observed, but one 

 species, though this is exceedingly variable, and in its extreme conditions 

 may easily be mistaken for distinct species. I have not been able to resolve 

 the stride upon the present species sufficiently to estimate their number; 

 there are, however, indications in the colour of the dry valve which convince 

 me of their presence. 



Plate XXXVII. 303 and 303 fi. 



Genus 38. ACHNANTHIDIUM, Kiitz. 



Frustules geniculate, rarely united into a filament, free ; valves stri- 

 ated, unsymmetrical. 



This genus, which for the present includes several species of some- 

 what doubtful character, was constituted by Professor Kiitzing to 

 receive those forms which, with an evident resemblance to Achnanthes, 

 are rarely united into a lengthened filament, and have no stipes or 

 other attachment to a foreign body. The genus Diadesmis of Ehren- 

 berg is a near ally, and in regard to some of the minute forms, where 

 the geniculate character of the frustule is with difficulty discerned, it 

 is almost impossible to decide to which of the genera we ought to 

 refer our specimens. I have adopted Kiitzing's genus, because our 

 two most distinct species, viz. A. lanceolatum and A. coarctatum, 

 evidently belong to it. Cocconeis Thwaitesii, W. Sm. (see Synopsis 

 of British Diatomaceae, vol. i. p. 21), is another species, which has 

 been placed in the present genus by its discoverer, M. De Brebisson : 

 if it should prove to be a perfectly free form, this allocation must be 

 admitted, and the species described in a future edition as Achnan- 

 thidium flexellum, Breb. 



1 . Achnanthidium lanceolatum, Breb. Filament of numerous frus- 

 tules, from 2 to 100; valves elliptical; upper with a median line 

 and central nodule ; the nodule in lower (?) dilated at one side 

 into two processes, extending to the suture; striae 40 in '001". 



