KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 26. N:0 2. 



33 



as Grunow remarks, this name involves a tautology. For the forms with the puncta disposed in 

 transverse and oblique rows, I reserve the name Pleurosigma. In Pleurosigma I also include such 

 of the forms of the Grunowian genus Rhoicosigma, as have the same disposition of the strise as 

 the true Pleurosigma. I have also included in Pleurosigma the Uonkinise with decussating strise. 

 The generic name Staurosigma may be abolished as it was founded in 1860 by Grdnow for Ehren- 

 bergs Stauroneis Sigma, which is nothing but a frustule of P. Normanii var. fossilis (Perag. Monogr. 

 de Pleuros. p. 26). For the few asymmetrical forms of Pleurosigma Donkin proposed in 1858 

 (Trans. Micr. Soc. Vol. VI) the generic name Toxonidea. I felt at first inclined to include these 

 forms in the genus Pleurosigma, but as such a change would be of little importance and the name 

 Toxonidea is so generally in use, I have decided to retain the latter genus. 



Pleurosigma, as here defined, comprises a large number of closely connected forms and is 

 not nearly related to any known genus, with the exception of Toxonidea. Among the Naviculse 

 some ievf forms (N. Placenta Ehb., N. Quincunx Cl.) have the same disposition of the alveoli, but 

 in other respects they are diff'erent. The same disposition of the alveoli is found also on the keel 

 of Amphiprora gigantea, and in a few Mastogloise. 



All true Pleurosigma-forms are marine. A few are pelagic in their habits. They occur in 

 all parts of the' world. 



The distinction of species is a matter of difiiculty. Grunow has in his monograph (Arct. 

 Diat. 1880) used as characteristics for the groups the angle at which the oblique rows of puncta 

 cross each other. Peragallo (Monographie du genre Pleurosigma, Diatomiste 1890 — 91) has adopted 

 the same method, which I think cannot well be maintained as a natural arrangement. 



Artificial key. 



1, 



S. 



3. 



4. 



5. 



6. 



7. 



8, 



9. 

 10. 

 11. 

 IS. 

 13 

 14. 

 16. 



f Valvi 



Valve very slightly or scarcely sigmoid . 

 sigmoid — 



{Median line straight, central 

 — — sigmoid — 



( Ends rostrate . . 



not 



{Median oblique striae more distant than the others . 



{Transverse and oblique striae equidistant .... 

 — striae closer than the oblique 

 J Valve rhomboid-lanceolate 

 \ — narrow linear-lanceolate . 

 ( Ends with a lunate mark 



■{■ 



^ without — — 



Valve lanceolate 



narrow, linear-lanceolate 



{Transverse striae 19 in 0,oi mm 

 — — as — ... 



{Median oblique striae more distant than the others 



{Valve narrow-lanceolate 

 — broadly 



/ Ends rostrate .... 



\ — not 



„ ) Ends acute . . 



\ — obtuse . . 



{Median line central 

 — — excentric 



f Valve 14 to 20 times longer than broad 



\ — 10 or less 



K. Sv. Vet. Akad. Hand. Band 26. N:o 2. 



.2. 



14. 



3. 



... .10. 



. P. cuspidatum Cl. 



4. 



.... P. nicobaricum Gkun. 



5. 



.... 6. 

 ... 7. 

 . P. directum Grun. 

 P. nubecula W. Sm. 

 . . P. Eudon Pant. 

 ... 8. 

 . . P. galapagense Gl. 

 9. 

 P. Peragalli Bbun. 

 P. ibericum Pek. 



11. 



12. 



. P. naviculaceum Bbeb. 



. . P. hungaricum Be. a. Cl. 



P. lanceolatum Donk. 



13. 



... P. pelagicum Pek. 

 . . . .P. marinum Donk. 



15. 



30. 



Clevei Gbtjn. (P. longum var.) 

 16. 



