164 E. M. NELSON ON THE NODULES IN PLEUROSIGM.E. 



and p. angulatum may be considered 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 extremities 

 of the valve, in the sweep of the curve of its raphae, in the excessive 

 coarseness of its perforated structure, and in the oval shape of 

 the perforations. The termination of its raphae pipes follows 

 P. angulatum, but the shape of the nodule is like P. ajjine var. 

 fossilis and the Maryland 2^leu7'osig7na. As diatoms are named 

 neither from the termination of their raphse 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 P. 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 curiously 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, but the 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 rapha? 

 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 (dry mount), nor in the N. firma var. Hitschcockii 

 (balsam), to be mentioned presently, although this last is a striking 

 object with an oil immersion g. With regard to the different 

 forms of nodules they may be divided into four groups. 1st, the 

 Maryland form (figs. 3 and 4). 2ndly, the angulatum form 

 (fig. 6). 3rdly, the Balticum form, Avhich outwardly resembles the 

 angulcotum form, but its raphse pipes enter farther into the 

 nodule, and they have distinct knobs * where they turn down into 

 the thickness of the silex, also the perforated structure is displaced 

 to the right and left of the nodule. 4th and lastly, we find a 

 nodule intermediate between the angulatum and Balticum forms. 



In the first group we have the Maryland Pleurosigma, affine, 

 affine Vdr. fossilis, rigidum, rhombeum, convexum, imviculaceum. 

 * The Cherry field Rhomhoides has somewhat similar knobs. 



