KONGL. SV. VET. AKADEMIRNS nANDLINGAE. BAND 26. N:0 2. 



23 



The forms of Tropidoneis have boat-like valves, more or less strongly keeled. The keels are 

 straight (not sigmoid) and either central (Orthotropis), or excentric. In the latter case they are 

 either turned in opposite directions {Plagiotropis), or in the same direction (Amphoropsis). To treat 

 these groups as different genera seems to me not to be natural; as the excentricity of the keel 

 is subject to variation. On one or both sides of the keel there is frequently an expansion, or wine/, 

 which in the valvular view has the appearance of a longitudinal line. On the broader side of the 



Tropidoneis concerta, 600 times magniiied. 



Tropidoneis Lepidoptera, 500 times 

 . magnified. 



valves of Plagiotropis there is frequently a longitudinal band, the nature of which is not quite 

 clear to me. It may be a wing, or a crest, as in many Nitzschiese, or perhaps a furrow. 



Tropidoneis' has evidently a close affinity with the Nitzschise of tbe Grunowian divisions 

 Bilobatse and Pseudo-amphiprora. Among the Kaphidiese it may be akin to the carinated forms of 

 Gryrosigma, or the so called Donkinise. It is also closely related to the Naviculse of the section 

 fusiformes which, although their valves are not keeled, agree with Tropidoneis in the nature of 

 the sculpture of their values, in then non-complex zones, and in thei approximate central pores of 

 their median lines., 



The species of Tropidoneis do not inhabit fresh water. Forms of the section Plagiotropis 

 are very frequent in brackish water. The large, wingless forms of Orthotropis . are pelagic in their 

 habits, and the winged, marine. They occur in all parts of the world from the arctic seas to the 

 tropical, 'iifi ■'.»>'•' -s"**/ 



*f. The forms of Tropidoneis are in many cases very imperfectly known, so that several species 

 are to be regarded as doubtful. The study of them is difficult, especially as regards the nature of 

 the wings, which require a favourable position of the valves for their examination. 



Artificial key. 



I. Orthotropis, median line central or nearly central. 



^ j Valve without wings ;. 



t — witli — 



I Valve moderately high 



\ — very high . . .;■..■':■. • . 



