168 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



forms with double central rosette, heteroporum to triangular forms 

 with single central rosette, galapagense to triangular forms with 

 no rosette, and finally the unnamed biangular form here illus- 

 trated, showing all the characteristics of the others except the radi- 

 ating watery lines that are generally but not always present in some 

 of them. As to whether or not these so-called species should be 

 retained for the greater convenience of future identifications depends 

 wholly on what one considers a diatom species to be. I have decided 

 to leave these specific names in my list of Philippine diatoms, as 

 offering better facilities for references to illustrations in other works, 

 but with the above-expressed opinion that they are in reality only 

 varied phases of T. eulensteinii Grunow. 



As to the confusion caused by some authors assigning these forms 

 to Trigonium (that is to say, the old Triceratium) and others to 

 Stictodiscus, a study and comparison of a liberal number of specimens, 

 examined in both valve and girdle aspect, will convince anyone that 

 Grunow, Van Heurck, Janisch, Cleve, and Schmidt are correct in 

 their original assignments to Triceratium, rather than to Stictodiscus. 

 It is probable that the watery radiating lines running from the margin 

 toward the center of the valve in some of the foregoing specimens 

 are responsible for their being classified under Stictodiscus, that genus 

 being characteristically marked with such lines. But Stictodiscus 

 always has a distinct border or rim, well defined and differently marked 

 from the rest of the valve, corresponding to the rim of AracJinoidiscus, 

 the genus with which Stictodiscus is most closely affiliated. No such 

 rim exists in any of the forms here under consideration. It is true 

 that something like a rim appears in some of the illustrations here 

 referred to; but in every case it is an illusion due to the appearance 

 of the beading at the edge of the valve, where it bends downward 

 toward the girdle, and a slight change of focus of the microscope 

 will quickly dissipate this false impression. A girdle view even 

 more clearly shows there is no trace of a rim ; and the whole structure 

 of the diatom seen in that aspect is so utterly unlike Stictodiscus 

 that no doubt remains. Such a view of a typical triangular 

 "Stictodiscus oicoronatus " Castracane, plate 37, figure 4, of this report, 

 will make clear the true structure. 



It may be added that the attempt to avoid the unsatisfactory classi- 

 fication of T. eulensteinii as a Stictodiscus by referring it and some 

 of these other forms to Pseudo-Stictodiscus is most unfortunate; so 

 also Van Heurck's suggestion of its being a Biddulphia (Treatise, 

 pp. 466, 468). The type species of the former is Pseudo-Stictodiscus 

 angulatus Grunow (Schmidt, Atlas, pi. 74, figs. 24-30), a diatom 

 that has no relationship with the present forms. H. H. Chase, (New 

 and Little-known Diat., p. 6) remarks upon the transfer of T. eulen- 

 steinii into Pseudo-Stictodiscus. "This splitting of well-established 



