132 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



to be consistent, it would require the subdivision of both Donkinia 

 and Khoicosigma into two genera each, as both contain species with 

 transverse-longitudinal lines as well as other species with three direc- 

 tion decussating lines. Although, as above mentioned, this genus 

 stands out sharply defined, the simple and unique uniformity of its 

 structure renders clear-cut demarcation between its species a difficult 

 thing. For we are confined to only three factors for our specific marks 

 of distinction — size, a very unstable and misleading quality; differ- 

 ence in contour, as relative width, sharpness or bluntness of apices, 

 etc.; and lastly, fineness or coarseness of marking. In consequence 

 of this, species have been unduly multiplied, and, on the other hand, 

 cautious students have grouped with known species some new forms 

 that should be held specifically distinct. In short, Pleurosigma in its 

 species represents one of the most difficult subjects in diatom taxon- 

 omy. One has, so to speak, to sense the species as one recognizes 

 individuals among our fellow men. Some distinctions are of course 

 easily made ; one can say at a glance that P. formosum W. Smith is 

 not P. angulatum W. Smith, that P. balticum W. Smith is not P. littor- 

 ale W. Smith. But many cases of classification in Peragallo's admir- 

 able monograph on this genus will continue to be approved or rejected 

 by different investigators. The several new species here described and 

 figured will appear to some as unimportant phases of some already 

 established. But in no case has a new species been created here with- 

 out careful study and an effort to assign it to an existing one. 



PLEUROSIGMA ACUS, new species 



Plate 29, figs. 4-5 



Valve very narrow lanceolate, straight; ends sharp; markings 

 quincunx, tranrerse lines obscure; terminal beads of rhaphe prominent; 

 hyaline space around central nodule small. 



Length 0.162-0.258; width 0.012-0.022; 13.5-13.8 lines in 0.01 mm. 



P. acus has close affinity with P. intermedium W. Smith, not so 

 much with the typical form as given in Smith's British Diatoms, 

 page 64, plate 21, figure 200, in Van Heurck's Synopsis, plate 18, 

 figure 6, in H. L. Smith's Types No. 405 (itself an English sample), 

 etc., as in certain perhaps permissible variations, notably, plate 5, 

 figure 27, in Peragallo's Monographic du genre Pleurosigma. But P. 

 acus, though a much smaller diatom, averaging 0.170 by 0.016 mm., as 

 against 0.370 by 0.021 mm., is relatively much more coarsely marked, 

 13.5-13.8 lines as against 22 lines in 0.01 mm., and the two oblique 

 lines are so nearly at right angles, instead of at 60°, that the trans- 

 verse lines are barely discoverable. Abundant specimens having 

 supplied an opportunity for repeated comparisons between this and 

 P. intermedium I do not hesitate to class it as a new species. All my 

 Philippine specimens are from Jolo Jolo, Sulu Islands; but I have 



