1S46.] Linneun Society. 311 



paradoxa, Gmelin." By G. H. K. Thwaites, Esq. Communicated 

 by William Spence, Esq., F.R.S., F.L.S. &c. 



Mr. Thwaites commences his memoir by a description of the spe- 

 cies. The filaments are ribbon- shaped, curved, pale brown with a 

 paler mesial line, and consist each of from 4 or 5 to upwards of 100 

 linear frustules, lying contiguous and parallel to each other. The 

 front view of each frustule exhibits a slight tapering towards the 

 apices, and a minute dentation on the inner edge of the smooth 

 raised lateral margins, the intermediate space being smooth. In a 

 side view, the surface of mutual contact is seen to be linear-lanceolate 

 with the apices rounded ; and the space between the smooth raised 

 lateral margins is marked with transverse raised striae. The length 

 of the frustule, and consequently the width of the filament, varies 

 from ^f^o to j^Q of an inch ; and the width of the frustule from 

 ru^oo ^^ J Goo ^^ ^^ iach. The frustules are filled (with the ex- 

 ception of a lighter transverse central fascia) with a pale brown en- 

 dochrome ; and the filaments increase in length from multiplication 

 of the frustules by fissiparous division. 



Mr. Thwaites has found this (the original) species of Bacillaria 

 abundantly in ditches at the mouth of the Avon near Bristol, in water 

 probably slightly bjackish, and also in small quantity in the river 

 Frome at Stapleton, encrusting various aquatic plants with a dark 

 brown coating, which under the microscope is resolved into a num- 

 ber of pale brown filaments that seem to adhere tolerably firmly to the 

 plant on which they are situated. When they have been for a few mo- 

 ments detached, 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 frus- 

 tule, the second acts simultaneously in a similar manner with regard 

 to the third, and so on throughout the whole filament ; the same 

 action having been going on at the same time at both ends of the 

 filament, but in opposite directions. The central frustule thus ap- 

 pears to remain stationary or nearly so ; while each of the others has 

 moved with a rapidity increasing with its distance from the centre, 

 its own rate of movement having been increased by the addition of 

 that of the independent movement of each frustule between it and 

 the central one. This lateral elongation of the filament continues 

 until the point of contact between the contiguous frustules is re- 

 duced to a very small portion of their length, when the filament is 

 again contracted by the frustules sliding back again as it were over 

 each other ; and this changed direction of movement proceeding, the 

 filament is again drawn out until the frustules are again only slightly 



