KONGl.. SV. VET. AKAUEMIENS HANULINGAK. BA.ND 26. N:0 2. 77 



preaches the central nodule. In other forms, as in D. lesinennis, they form several, more or less 

 irregular, longitudinal rows. The ocelliferous stratum seems to correspond to the interior porous 

 layer in Trachynein and perhaps to the foramina of the larger Finnularia. 



Diploneis has, as already remarked, some affinity to the group of Navieulee lyratee, inter- 

 mediate forms being D. hyalina and D. Hudsonis. It has also some relation to Cymatoneis and 

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

 phora, sensu strictiori. In the last named group we find the large central nodule, the lateral 

 furrows, and, frequently, the reticulated structure of some Diploneis-forms ; but there are no known 

 intermediate forms between Diploneis and Amphora. 



The systematic arrangement of the numerous forms of Diploneis is excedingly 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- 

 ration. Several »new species* have been founded on corroded specimens of well-known species. 

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

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

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



Most species of Diploneis live in salt water. Brackish forms are 1). didyma, D. interrupta, 

 I). Sniithii and frequently D. elliptica. The panduriform species are all marine or brackish, and 

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

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



Artificial key. 



^ I Finely strate (striae 17 to 24 in 0,oi mm.) . . . . . . 2. 



\ Coarsely — (costae 3 to 17 in 0,01 mm.) . ... G. 



„ I Striae of distinct, coarse puncta . . . . D. ovalis Hilse. 



\ — not distinctly or finely punctate 3. 



„ ) Horns of the central nodule strong and distinct] 4. 



' ] — — — • — not distinct ... . . . . . 5. 



'■{' 



Transverse striae crossed by oblique striae . . D. Hudsonis Grun. 



— not — — . D. hyalina Donk. 



f Furrows broad . . . D. bioculata Geun. 



( — narrow D. oculata Breb. 



I Transverse costae not crossed by longitudinal, or not alternating with alveoli . 7. 



6. < — — crossed by one or several longitudinal, or alternating with single 



I or double rows of alveoli . ... . 25. 



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



' I — — — — distinct . . ... . . 8. 



J, I Furrows narrow . . . 9. 



\ — broad . ... .... 18. 



„ J Valve constricted . ... 14. 



\ — not or slightly, . . . . . . . 10. 



[ Valve elliptical . 11- 



10. ■! — linear-elliptical ... . . 12. 



I — linear . .... ... ... .... 13. 



f Costae 6 to 10 in 0,oi mm. . . . . D. suborbicularis Greg. {D. coffmformis A. S.). 



Jl- \ _ 12 to 18 — D. Puella A. S. 



( Fresh-water species . . ... D. Boldtiana Cl. 



■ \ Marine . . . . .... J), advena A. S. 



I Horns parallel . D. congrua Jan. 



■ \ — divergent in the middle . ..... ■ B. compar Jan. 



^ . i Slightly constricted . . . . . . D. subnuda A. S. 



\ Strongly — .... 15. 



( Ends rostrate . . ... . . . . D. laciniosa A. S. 



^^- \ - not - . . ■. 16. 



