6 DR. GREGORY, ON SOME NEW SPECIES 



36 in '001. I name this and the preceding species with some 

 hesitation, not that they are not well-marked forms, as may be 

 seen from the figures, but because the genus Cymhella^ as well 

 as the allied one Cocconema, is not in a satisfactory state, and 

 requires a thorough investigation, in which the forms I here 

 describe must be considered. The same remark applies to 

 the next species. 



20. Cymhella Piscicuhis, W. G. — This form occurs in a 

 very interesting gathering from Norfolk, and Dr. Greville has 

 recently found it near Bridge of Allan, I have also lately 

 seen it in various gatherings, including that from Lochleven. 

 It is rather large, broad, and has somewhat square apices. 

 Length about "0016. Striae about 30 in •100. 



21. Cymbella Arcus, W. G. — This pretty form I have very 

 recently found in two gatherings from the neighbourhood of 

 Hamilton. The ventral surface is straight, the dorsal highly 

 arcuate, and slightly undulating, broad in the middle, very 

 narrow towards the extremities, like a strung bow. The apices 

 are rather square, expanding a little, after a slight construc- 

 tion. The striae are best seen about the middle, where the 

 frustule is broadest. Length about -0014. Striae about 30 in 

 •001. 



22. Navicula cocconeiformis, W. G. — Occurs in Elgin, 

 Elchies, and some other Banffshire localities, and Lochleven, 

 and recently in various gatherings from different parts of 

 Scotland. In form it is short, broad, nearly oval, but with 

 a slight angularity in the middle, and flattened apices. Some 

 specimens are almost rhombic. In shape it comes very near 

 to Cocconeis Jiexella ( Tfiwitesii), and it has much the aspect 

 of that form, except that the median line is quite straight. 

 Striae not resolvable. Length from "0006 to '0012. I under- 

 stand that ttiis form has been named N. nugax by Professor 

 Smith, but I consider my own name, given much earlier, as 

 more characteristic. Besides this. Dr. Greville has lately 

 figured it under the name here adopted. 



23. Navicula lacustris, W. G. — This fine species has only 

 as yet occurred in the gathering from Lochleven, in which, 

 though not abundant, it is yet far from scarce. It presents 

 two well-marked varieties, a and /3, which pass into each other 

 by intermediate forms : «, which is rather more abundant than 

 the other, is elliptico-lanceolate, with acute apices. Nodule 

 bright in the centre, but without definite outline. Median 

 line double. Striir fine, but distinct, slightly inclined ; about 

 28 or 30 in. -001 ; length from -0016 to '0025 inch : ^ agrees 

 in every point with a, except in outline. It is broad, has 

 straight sides, sometimes even a little incurvated, and sud- 



