OF BRITISH FRESH-WATER DIATOMACE^. 7 



denly contracted to narrow produced extremities. These two 

 varieties are seen in figs. 23 and 23 iS, and there exists an 

 intermediate form. The only species with which this could 

 in any way be confounded is iV. Jirma, var. ^. But the 

 latter is longer and larger, always of a brown colour, and in 

 N. firma not only are the striae much finer and less con- 

 spicuous, but they are almost exactly parallel. It is not easy 

 in a drawing to give certain peculiarities of asp?ct, but any 

 one who compares the two species, N. firma (3 and N. lacustris, 

 will perceive that the latter has an aspect entirely different 

 from the former. JVIoreover the side lines, always seen in 

 N. firma, never occur in N. lacustris. 



24. Navicula bacillaris, W. G. — This pretty little species 

 was first observed in several gatherings from Duddingston 

 Loch, and has since occurred in many others, as Lochleven, 

 Elchies, Elgin, and in large quantity in two from the neigh- 

 bourhood of Dundee. It is linear, narrow, with rounded and 

 slightly pointed apices. It has a very smooth aspect, and the 

 striae are so fine that it is difficult to resolve them. Length 

 from -0012 to -002 inch. 



25. Navicula lepida a, W. G. — This form occurs in the 

 Lochleven gathering, where it is not rare, and I have recently 

 seen it in others, as in those from Hunter's Bog, and in 

 one of Dr. Balfour's from Borthwick Castle.* It is small, 

 of a narrow oval, and has at first sight a smooth polished 

 aspect ; but on closer inspection the striae are seen to be by 

 no means very fine, but rather difficult to catch from their 

 transparency. They ai'e distinctly but slightly radiate. The 

 central nodule is indefinite, and assumes frequently the aspect 

 of a hazy cross-band, approaching to that of a false stauros. 

 It is a very neat and well-marked little form, and its characters 

 are very constant, varying considerably only in length ; the 

 average being about 'OOIS inches. 



25/3. Navicula pileda ? /6, W. G. — This is another small 

 form, which in some points is so near the last, that I regard it 

 for the present as a variety of N. lepida. It is small, of a very 

 short oval shape, and the striae are both more distinct and 

 more radiate than in N. lepida, but yet it exhibits at first 

 sight the same apparent absence of striation. The difference 

 is in the structure of the nodule, which in the present case is 

 always very definite, transparent and glassy ; the same peculiar 

 aspect is seen in the terminal nodules ; this form hardly ever 

 varies even in length. I have only once seen a specimen a 

 good deal longer and narrower in proportion, and consequently 



* Dr. Greville has also found it iu a recent gathering from Duddingston 

 Loch (April, 1855). 



