KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 26. N:0 2. 33 



as Grunow remarks, this name involves a tautology. For tlie forms ■with the puncta dispos(id in 

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

 of the forms of the Grunowian genus Rhoicosi(jnut, as liave the same disposition of the stria' as 

 the true Plextrosigma. I liave also included in Pleurosigma the Donkiniit' with decussating stria'. 

 The generic name Staurosigma may be abolished as it was founded in 18G0 by Grunow for Ehren- 

 bergs Staiironeis Shjma, which is nothing but a frustule of P. Normanii rar. 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 Toxonidca. 1 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 Navicixla', 

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

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

 of Amphiprora (jigantea, and in a few Mastogloice. 



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 ol)lique 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 hey. 



^ i Valve very slightly or scarcely sigmoid 2. 



\ ■ — sigmoid — • 14. 



„ j Median line straight, central 3. 



I — — sigmoid — 10. 



„ I Ends rostrate P. cuspidatum Cl. 



1 — not 4. 



, J Median oblique striae more distant than the others P. nicoharicum Grun. 



' I — — — not — — — 5. 



p. ) Transverse and oblique strise equidistant 6. 



1 — strise closer than the oblique 7. 



^ j Valve rhomboid-lanceolate P. directum Grun. 



t — narrow linear-lanceolate P. nvbeciila W. Sm. 



j Ends with a lunate mark . . . .* P. Eudon Pant. 



{ 



Transverse striae 



"2'^ — p. ibericum Per. 



7. . 



without 



„ J Valve lanceolate P. g(dnpagvnf)C Cl. 



narrow, linear-lanceolate 9. 



^. ) Median oblique striae more distant than the others 11. 



„.{ 



I — — not — — — 12. 



Valve narrow-lanceolate P. naviculaccum Breb. 



— broadly P. hungaricum Br. a. Cl. 



^o ( Ends rostrate P. lanceolatum Donk. 



I — not 13. 



^„ I Ends acute P. pelagicum Per. 



\ — obtuse P. marinuni Donk. 



.J . f Median line central 15. 



I — — excentric 30. 



..p, f Valve 14 to 20 times longer than broad P. Clevei Grun. (P. longum var.) 



' 1 — 10 or less 16. 



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



