OP THE GENERA NAVICULA AND CYMBELLA. 417 



-very true to nature. It seems to be undoubtedly Cymhella 

 ■angustata. 



Further, according to Cleve and to van Heurck, it has been 



somehow mixed up with Cymhella aequalis or suhaequalis, and 



;that seems more inexplicable, seeing that the so-called aequalis 



•group, although very akin to angustata, is yet different enough. 



Cymhella angustata is undoubtedly a form steadily passing, 

 ^perhaps through the aequalis group, into the American Cymhella 

 . Stodderi — a living form characterised by rostrate or subrostrate 

 -ends, broader areas and punctate striae. 



Cymbella aequalis and subaequalis (Plate 27, figs. 7,8). 



These two forms are so near to each other that it is almost 

 impossible to distinguish them, and indeed Cleve simply unites 



"them together into one species. There seems to be a difference 

 between them only in the dimensions of the axial area and perhaps 

 in size. Both these forms have the same chief specific feature, 



■which is that the median striae are farther apart (decipient). 

 Both are rather distinctly asymmetrical, and the ends are rather 



-obtuse. Both are common in Europe. I should say that in 

 America they pass into Cymhella Stodderi, but their striae are 



.always much less distinctly punctate. The aequalis group is very 

 numerous in the St. Fiore slide, where it shows some insignificant 



•variations. One of such varieties has been styled by Grunow 

 var. Florentina, but it seems hardly to deserve a special name. 

 All these forms are small, not more than 0*04 mm. in length. 



'The striation is not so dense as in the American forms. The 

 w^hole group seems to be very akin, on one side to the angustata 

 group, on the other to Cymhella Stodderi. I have therefore placed 

 the aequalis group in the genealogical table as intermediate be- 



"tween the angustata and the Stodderi groups. 



Cymhella Stodderi (Plate 27, figs. 5, 6). 



This species is most interesting from our point of view. It is 

 "to be found on the Herkimer slide in profusion. Cleve states 

 "that it also inhabits the White Mountains. It is nearly symme- 

 ■trical, yet still a Cymbella. It has subrostrate ends. The striae 

 are very punctate, radiate, numbering about 15 or 16 to O'Ol mm. 

 I cannot understand why Cleve says that it is not distinctly 

 j^-punctate. On the contrary it is, at least on the Herkimer speci- 



