66 SUMMARY OF CURRENT RESEARCHES RELATING TO 



generally held opinion that Lbe girdle-band of diatoms is a " closed 

 hoop" is erroneous in a large number of instances, with certain impor- 

 tant exceptions however. The authors, in fact, state that " the closed 

 hoop structure is unusual." " The girdle is a two-ended band of silica 

 with the ends variously and characteristically rounded or otherwise 

 modified and approximated or overlapping without being joined," at least 

 in certain examples amongst the larger forms of, e. g. Surirella, Nitzschia, 

 Navicula, &c, and inferentially in the minuter species of the same genera ; 

 and also in certain species of Coscinodiscus, Aulacodiscus, Bidduljahia, &c. 

 Forms with closed girdles are Synedra supcrba, Arachnoidiscus Eltren- 

 bergii, Triceratium favus and probably all Triceratiinse, Isthmia nervosa. 

 The pertinence of these observations to the phenomena of reproduction 

 and development is obvious, and they will serve to clear up some at least 

 of the difficulties involved in the assumption of a closed fixed hoop. The 

 important excej)tions, however, seem to leave the problem much as before, 

 although the paper is confessedly only a preliminary one. The struc- 

 ture called by the authors a "cleat "on the secondary girdles which 

 make their appearance before reduplication, and which is particularly 

 well marked in Surirella clegans Ehr., is very interesting and, so far as 

 we know, new. The paper is accompanied by two plates of diagrams 

 and photographs. 



Actinoeyclus Ralfsii. * — E. M. Nelson points out that diatoms in 

 general appear white, when examined by a low power and ordinary 

 •transmitted light upon a light ground ; but if the light ground be made 

 dark, by placing a central stop beneath the condenser, those diatoms 

 which have compai'atively coarse structure will appear red, those with 

 finer structure green, those still finer blue, and so on. He notices that, 

 these colours are due to diffraction, and that if the objective be changed 

 for one of greater aperture, those diatoms which were red will be re- 

 solved and become colourless, those that were green will become red, 

 those which were blue green, and the whole series lowered one step in 

 the gamut of the spectrum. 



He then calls attention to the very beautiful Actinocycli, which act 

 in a manner precisely contrary to the above ; for upon a dark ground 

 they appear colourless, while upon a light ground they become brilliantly 

 coloured. Mr. Nelson argues that the colours in this species can be 

 caused neither by diffraction nor by pigments, and he asks for some 

 explanation of the phenomena. 



In conclusion, he describes an extremely delicate perforated cap or 

 sieve covering the single process situated near the margin of this diatom, 

 and says that it is similar to the caps of the processes of the Aulisci, to 

 which he drew attention on a former occasion. 



Schmidt's Atlas der Diatomaceen-Kunde. — Heft 5G of this magnifi- 

 cent work comprises, as usual, 4 plates, Nos. 221—224, with accompanying 

 brief descriptions. The diatoms figured are species of llhabdonema and 

 Cyclotella. 



Stigeoclonium. j — L. Iwanoff finds, in the neighbourhood of Moscow, 

 a new terrestrial species of Stigeoclonium, which he names S. terrcslre, and 



* Journ. Quek. Micr. Club, \\l (1900) pp. 377-8J. 



f Bull. Soc. Imp. Nat. Moscou, 1899 (I'JOJ) pp. 423-32 (1 pl^ (German). 



