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



as GrRUNOw remarks, this iiame involves a tantology. For the forms with the puiicta disposed in 

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

 of the forms of the Grunowian genus Ilhoicosigma, as have the same disposition of the striiE as 

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

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

 bergs Sfauroneis Siyma, which is nothing hut a frustule of P. Normanii var. fossilis (Perag. Monogr. 

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

 (Träns. Micr. Soc. Vol. VI) the generic name Toxonidca. I felt at iirst inclined to include these 

 forms in the genus Pleurosigma, but as such a chauge 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 deiined, comprises a large number of closely coniiected forms and is 

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

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

 in other respeets they are different. The same disposition of the alveoli is found also on the keel 

 of Amphiprora giganfea, and in a few Mastogloiaä. 



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

 all parts of the world. 



The distinction of species is a matter of difRculty. Grunow has in his monograph (Aret. 

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

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

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



Artificial key. 



^ j Valve very slightly or scarcely sigmoid 2. 



\ — sigmoid — 14. 



„ j Median line straight, central 3. 



' \ — ■ — sigmoid — 10. 



_ I Ends rostrate P. ciispidatum Cl. 



I — uot 4. 



. ) Median oblique striEe more distant than the others P. nicoharicum Grun. 



\ — — — not — — — 5. 



j Transverse and oblique striae equidistant 6. 



I — strise closer than the oblique 7. 



. I Valve rhomboid-lanceolate P. äirechiin Grun. 



\ — narrow linear-lanceolate P. nubecula W. Sm. 



_ J Ends with a Innate mark P. Euäon Pant. 



\ — without — — 8. 



„ j Valve lanceolate P galapagcnse Cl. 



\ — narrow, linear-lanceolate 9. 



Transverse strise 19 in 0,oi mm P. Peragalli Brun. 



I' 



9. 



ys — P ibcricum Per. 



.,„ ) Median oblique stris more disfant than the others 11. 



^"- 1 - - - not - - - 12. 



^.| (Valve narrow-lanceolate P. iiaviculamini Bréb. 



\ — broadly P himgaricnm Br. a. Cl. 



^„ j Ends rostrate P. lanceolattim Donk. 



1 — not 13. 



..„ I Ends acute P pelagiciim Per. 



\ — obtuse P. marinum Donk. 



... ( Median line central 15. 



' \ — — excentric 30. 



„ ( Valve 14 to 20 timas longer than broad P. Clevei Grun. (P longum var.) 



\ — 10 or less 16. 



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



