1899] MICROSCOPICAL JOURNAL. L93 



the centre and curves more rapidly at the extremities, 

 while in Pleurosigma affine var. fossilis, the Maryland 

 pleurosigma and Pleurosigma angulatum, the curve of 

 the raphae has a more regular sweep throughout. In fact, 

 these three diatoms — viz. P. affine var. fossilis, the 

 Maryland pleurosigma and P. angulatum may be consid- 

 ered identically the same as to their exterior shape, the 

 only differences between them being in the nodules and 

 in the coarseness of their perforated structures. P. affine 

 differs from these three in the bluntness of the extremi- 

 ties of the valve, in the sweep of the curve of its raphfe, 

 in the excessive coarseness of its perforated structure, 

 and in the oval shape of the perforations. The termina- 

 tion of its raphae pipes follows P. angulatum, but the 

 shape of the nodule is like P. affine var. fossilis and the 

 Maryland pleurosigma. As diatoms are named neither 

 from the termination of their raphae pipes, nor from the 

 shape of their nodules, but solely from their exterior 

 form, it is difficult to understand why the P. affine var. 

 fossilis should have been named a variety of P. affine 

 and not of angulatum. 



With regard to the perforated quincunx pattern all 

 over the valve, that on the Maryland Pleurosigma is 

 coarser than that on P. angulatum, but in other respects 

 it is very similar. That of P. affine is coarser than that 

 on any other Pleurosigma I am acquainted with, and cu- 

 riously the perforations are, as just stated, oval. That 

 on P. affine var. fossilis, however, is finer and less oval 

 than that on P. affine. The structure on the P. affine is 

 so coarse that it can be easily resolved by any lens of .45 

 N. A. aperture, with axial cone illumination, butthe same 

 lens will barely show striae on a P. angulatum with 

 oblique light. An apochromatic 4 mm. for the long tube 

 will show the bent raphae pipes in P. affine (balsam), P. 

 affine var fossilis (balsam), and in P. angulatum (dense 

 medium), but it will show neither those in P. angulatum 



