KUNGI>. SV. VKT. AKADEMIENS HANULINGAK. BANIi 26. N:u 2. 77 



proaches the central nudule. In other forms, as in D. lesinensin, they form several, more or less 

 irregnlar, longitudinal rows. The ocelliferons stratum seems to oorrespond to the interiör porons 

 layer in Trachyneis and perhaps to tlie foramina of the larger Fiiniularla. 



Diploneis has, as already remarked, some affinity to the group of Naviadce lyratce, inter- 

 mediate forms being D. hijaiina and D. Hiuhonis. It has also sonie relation to Cymafoneis and 

 through that genus to Scoliopleura. Another genus, wliich has some relation to Diploneis, is Am- 

 phora, sensu strietiori. In the last named group \ve find the large central nodule, the lateral 

 furrovvs, and, fret|uently, the reticulated structure of some Diploneis-forms; but there are no known 

 intermediate forms betvveen Diploneis and Amphora. 



The systematic arrangement of the uumerous forms of Diploneis is excediugly difficult. 

 The species are very variable in size and in the coarsness of the structure. Moreover valves, which 

 are uninjured often present a very dissimilar appearance to such as have been corroded by prepa- 

 ratiou. Several aiew species» have been founded on corroded speciniens of well-knowu species. 

 Many, at first sight very distinct species, are connected by intermediate varieties. Although I 

 have examined and tigured a very large number of forms from all parts of the world, I must 

 confess that my attempt to their classitication still leaves much to desire. 



Most species of Diploneis live in salt water. Brackish forms are I), didyma, I), interrupta, 

 D. Smithii and frequently D. dliptica. The panduriform species are all marine or brackish, and 

 D. didymu becomes almost elliptical in slightly brackish water. Fresh water species of Diploneis 

 are few, but very frequent. The are all elliptical in oiitline. 



Artificiell key. 



. I Finely stråte (strite 17 to 24 in 0,0 1 mm.) 2. 



I Coarsely — (costw 3 tu 17 in 0,0 1 mm.) ö- 



g ( Strife of distinct, coarse puncta . . D. ovalis Hilsb. 



\ — not distinctly or finely pnnctate 3. 



„ I Horns of the central nodule strong and distinct i- 



' ' ] — — — — not distinct o. 



, j Transverse striae crossed by obliqne striae D. Hudsonis Grun. 



' \ — — not — — D. hijalina Donk. 



I Furrows broad D. bioculata G.RCN. 



' ■ i — narrow D- oadata Bkéb. 



f Transverse costa; not crossed hy longitudinal, or not alternating with alveoli ... 7. 

 G. ■! — — crossed by one or several longitudinal, or alternating witli single 



\ or double rows of alveoli 25. 



1 Horns of the central nodule not distinct D. inscripta Cl. 



" \ — — — — distinct 8. 



( Furrows narrow 9- 



1 — broad . . ■ 18. 



,- I Valve constricted 14. 



\ — not or slightly,^ 10. 



f Valve elliptical 11- 



10. '. — linear-elliptical 12. 



I — linear 13. 



) Costse 6 to 10 in O.oi mm D. stiborbkidaris Geeg. {D. coffcefoinns A. S.). 



I — 12 to 18 — D. Puella A, .S. 



) Fresh-water species D. BoldUana Cl. 



\ Marine D- advena A. S. 



I Horns parallel D. congrua Jan. 



1 — divergent in the middle D. compar Jan. 



I Slightly constricted D. suhnuda A. S. 



1 Strongly — 15. 



j Ends rostrate D. laciviona A. S. 



\ - not — 1<^. 



11. 

 12. 

 13. 



14. 

 15. 



