OF BRITISH FRESH-WATER DIATOMACEiE. 11 



the apex, the other on the opposite side of the opposite apex, 

 thus dividing the valve into two halves, which, although equal, 

 are so placed that the narrowest part of one corresponds to 

 the broadest of the other, as is well shown in the figures. In 

 some cases, as may be seen in the larger figure, the median 

 line is slightly sigmoid, but this is rare. The obliquity just 

 described, which I do not remember to have seen in any other 

 species, is invariably present ; at least I have found it in at 

 least 150 specimens wluch I have examined. The length is 

 from 001 to "0022 inch. Striae, by the measurement of Pro- 

 fessor Kelland, 45 in "001 inch. I may add that Professor 

 Kelland thinks the median line is twisted, as it were on its 

 own axis, to a certain extent, 



36. Stauroneis (?) ovalis, W. G. — This very pretty little 

 form first occurred to me in some gatherings made on the 

 River Findhorn by my friend Mr. Crawford, of Overton. In 

 one of these it is quite the predominating form, and in all of 

 them JV. incurva, already described, also occurs. I have 

 recently found it, much more sparingly, in Lochleven, in 

 which I also detected N. incurva. The form is a pure oval, 

 '001 inch long, and it is crossed by what at first I took for a 

 stauros, which is broad and reaches the margin. But 1 can- 

 not, with a high power, satisfy myself that this is really a 

 sfauros, as it seems to vanish, or is so transparent that it 

 cannot well be traced. The valve appears to be convex, as 

 when the stauros is brought into focus, the other parts are 

 but dimly visible. The stria' have not yet been resolved. 

 As the genus of this form is not yet determined, I retain the 

 name Stauroneis with a mark of interrogation. It is more 

 probable that it may prove to be a Cocconeis. At all events, 

 it appears to be a distinct and well-marked species. Length 

 about "OOl inch. I have recently observed it in two gather- 

 ings from Lanarkshire. 



37. Stauroneis dtibia, W. G. — This is a still smaller form, 

 and, as the name indicates, its true position is not quite 

 settled. It occurs in some of the gatherings from Dudding- 

 ston Loch, and in others from the Hunter's Bog, and is far 

 from scarce. It is small, narrow, of an elliptico-lanceolate 

 form, the apices slightly truncated. There is a stauros, 

 whether true or false is not yet ascertained, but probably 

 true. When examined under a high power, the valve exhibits 

 two parallel marginal lines within the margin on each side, 

 the stauros not reaching farther than the inner one of these 

 lines. The stria? have not yet been resolved. Length from 

 •0008 to -0012 inch. 



38. Surirella tenera? W. G. — This pretty form occurs in 



