KONGI.. SV. VKT. AKADKMIKNN II AN DLI N(i AH. HANI) 26. N:() 2. 77 



])i'()ax'lies the ceiiti'al nodule. In otlier tonus, as in 1). Ics/Neusi.s, they form several, more or less 

 irregular, longitudinal rows. ^Plie oeellit'erons stratum seems to correspond to the interior porous 

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



Diploneis has, as already remarked, some affinity to the group of Naviculcc^ li/ratrt', inter- 

 mediate forms being D. In/a Una and D. Iludsonis. It has also some relation to Cymatoneis and 

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

 phora, sensu strietiori. In the last named gi'oup we find the large central nodule, the lateral 

 furrows, and, fre(|uently, 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 s])ecies. 

 Many, at first sight very distinct species, are connected by intermediate varieties. Although 1 

 have examined and figured a very large number of forms from all parts of the world, 1 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). Sinlthii 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 fre(|uent. The are all elliptical in outline. 



Artificial key. 



. j FiiKily .st.rate (.stria 17 to '24 in 0,0 1 mm.) 2. 



\ Coarsely — (co.stip H tn 17 in O.oi mm.) ''. 



jj I Striae of distinct, coarse puncta 1>. oralis Hilse. 



1 — not distinctly or finely punctate -J. 



,, I Horns of the central nodule strong and distinct 4. 



1 — — — — not distinct 5. 



, I Transverse strise crossed by oblique stri* D. Hudsonis Grun. 



' \ — — not — — D- hyalina Donk. 



I Furrows broad D. hiocnlata Grun. 



) — narrow D. ondafa Breb. 



j Transverse cost* not crossed by longitudinal, or not alternating with alveoli . . . 

 > — — crossed by one or several longitudinal, or alternating with single 



7. 

 G. 



I or double rows of alveoli 25. 



_ J Horns of the central nodule not distinct D. inscripta Cl. 



\ — — — — distinct f^. 



I Furrows narrow it. 



' ■ i — broad 1». 



, ) Valve constricted 14. 



"1 — not or slightly, 10. 



f Valve elliptical 11. 



10. I — linear-elliptical 12. 



I — linear 13. 



. j Costa? 6 to 10 in O.oi mm 1). siihorhicularis Greg. (/>. roffwformis A. S.). 



I Costa? 

 \ - 1 



12 to 18 — D. Pndla A. S. 



.„ ) Fresh-water species D. Boldtiana Cl. 



1 Marine D. adretia A. S. 



.„ ) Horns parallel Z). congrua Jan. 



' — divergent in the middle D. cotnpar Jan. 



Slightly constricted D. subnuda A. S. 



Strongly — 15. 



j Ends rostrate D. Inriviosn A. S. 



"I 



not — , Kj 



