BRITISH DIATOMACE^E. 



In mode of growth and form of filament exactly like the last. I have never 

 been able to verify the description given by Mr. Ralfs on the authority of 

 Mr. Jenner, and copied by succeeding writers, that " the frustules stand erect 

 like the staves of a tub, and when dry fall down and necessarily separate." 

 In the specimens I have gathered in Westmoreland, and those received from 

 M. De Brebisson, the circles are invariably arranged on a plane as in the last 

 species ; and although the adherence of the valves of the contiguous frustules 

 appears less firm than in that species, there is no lack of filaments exceeding 

 a semicircle, and not a few forming a complete round. 



In my specimens the present is generally a smaller form than the last, and 

 the F. V. of the frustule broader in proportion to the length. The same 

 curious anomaly in the mode of growth is found in var. /3. as noted under 

 M. circulare, an additional proof that such peculiarities ought not to be 

 adopted as a specific distinction. Var. y. of our present form is probably a 

 sporangial state of the plant, which escaped my recognition in the freshly 

 gathered specimens. 



Plate XXXII. 278. Var. (3. 278 /3. Var. y. Supp. PI. LX. 278 y. 



Genus 31. BACILLARIA, Gmel. 



Frustules linear, straight, united into a filament, at first attached, at 

 length free, coiled inwards ; valves elongate, keeled, with a longi- 

 tudinal line of puncta, keel eccentric. 



The frustules of Bacillaria are those of a Nitzschia; but their 

 adherence after self-division into a filament, frequently of considerable 

 length, prevents the observer from placing them in the latter genus. 

 When observed in a growing state, there is another peculiarity which 

 at once serves to discriminate our native species, and that is the 

 extraordinary motion with which the united frustules are endowed, 

 and which is not seen in any other of the filamentous Diatomaceae. 

 This movement has been so well described by Mr. Thvvaites in a 

 communication to the Linnean Society (Proceedings, vol. i. p. 311), 

 that I shall avail myself of his observations in the elucidation of the 

 subject. Mr. Thwaites remarks that, " When the filaments have been 

 detached from the plants to which they adhere, a remarkable motion 

 is seen to commence in them. The first indication of this consists in 

 a slight movement of a terminal frustule, which begins to slide 

 lengthwise over its contiguous frustule ; the second acts simul- 

 taneously in a similar manner with regard to the third, and so on 



