KONGL. SV. VET. AKADEMIKNS HANDLINGAR. BAN 



D 26. 



n:o 



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 direetions (Plagiotropis), or in the same direction {Amplioropsis). To treat 

 these gronps as different genera seeras to me not to be natural; as the excentricity of the keel 

 is snbjeet to variation. On one or both sides of the keel there is freqnently an expansion, or winri, 

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



Tropidoneis concerta, MO times magnifled. 



v _J_^;1I_^>^ 



Tropidoneis Lepidoptera, fiOO times 

 magnifled. 



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

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



Tropidoneis bas evidently a close affinity with the Nitzschia; of the Grunowian divisions 

 Bilobatte and Pseudo-amphiprora. Among the Raphidiese it may be akin to the carinated forms of 

 G-yroisigma, or the so called Donkinige. It is also closely related to the JSTavicnliB of the section 

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

 the sculpture of their values, in then non-complex zones, and in the 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. 



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 diiHcult, e.specially as regards the nature of 

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



Artificial kcy. 



I. Orthotropis, median line central or nearly central. 



I Valve witliout wings 2. 



■ \ — witli — 5. 



g I Valve moderately high T. adriatica Cl. 



"' \ — very liigli 3. 



