28(5 SUMMARY OF CURRENT RESEARCHES RELATING TO 



flat disks closely adprcsscil to stoucs beneath the surface of the water, 

 15 era. in diameter, and with the thallus often as much as 20 cells in 

 thickness. In its mode of growth and cell-division it presents a 

 remarkable resemblance to Coleochcete, but the cells are filled with a 

 black-brown endochrome. Lithoderma differs from Balfsia in the 

 sporangia being developed directly on the surface of the thallus, 

 instead of being more or less included within it. M. Flahault found 

 in L. fontanum only unilocular zoosporangia, and not the two kinds 

 described by Areschoug in the genus. The zoospores germinate 

 directly, and show no tendency to conjugate. 



Structure of the Diatom Valve.* — R. P. H. Durkec, referring to 

 Dr. J. D. Cox's view that each valve consists of two laminre connected 

 by a wall-like structure, in its outline sometimes hexagonal, at others 

 circular, and that each plate is whole or imperforate, says that while 

 engaged in the examination of a preparation of the Nottingham earth 

 he came across a fine specimen of Heliopelta. The surface being 

 undulating, the focus so happened to be arranged as to sharply define 

 the highest points. Changing the focus, he became aware of a fine 

 crack beginning at the lower margin of the valve, and extending over 

 the segment and running parallel with one of the points of the star 

 to its layaline centre, and he found the crack divide and throw out 

 two branches, completely crossing the centre and following out their 

 course over the opposite segments. The centre was perfect in its 

 framework except for the cracks, and the question whether there was 

 a film, and if so, a continuous surface, was answered. 



As further evidence of the existence of the two plates, the author 

 says that Dr. Detmers, in examining the specimen, clearly demon- 

 strated that the upper film only was cracked and the lower layer intact ; 

 and the paper is accompanied by photographs which have been so 

 arranged as to focus, that they exhibit this point. 



Structure of Diatoms.f — G. C Wallich refers to Flogel's view| 

 that in such genera as Tricerafium and Coscinodiscus, the little hexa- 

 gonal or cylindrical cavities, though completely closed by a siliceous 

 film on the internal surface of the valve, are not closed by any such 

 membrane on the outer surface of the valve ; and to Cox's, who insists § 

 that the cellules are closed by a siliceous film externally as well as 

 internally. 



Dr. Wallich considers the objections to the latter view insuperable. 

 If the cellules are closed at both their extremities during the life of 

 the organism, each cellule must be full either of protoplasm or of some 

 other more or less fluid substance, unless, indeed, each contains a gas, 

 or constitutes a perfect vacuum, which is scarcely within the bounds 

 of possibility. If each contains protoplasm, it is obvious that the 

 remains of this, during the mounting of the specimen, would be 

 recognizable amongst the larger species, either by optical or chemical 



* Proc. Amer. Soc. Micr., 7th Ann. Meeting, 1884, pp. 105-9 (1 heliotype). 

 t Engl. Mech., xl. (1885) p. 496. 

 X See this Journal, iv. (1884) p. 665. 

 § Ibid., p. 941. 



