ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1025 



plate, and that these plates show no sign of orifices, hut only of 

 thinnings over the cavities, except in abnormal cases where the 

 organic cuticle has been partially or totally destroyed by accidental 

 causes. 



(d.) That the external membrane is in most cases so slightly 

 silicious that even slight contact with acids promptly destroys it, and 

 opens up the cavities at the back of it. That in other cases this 

 membrane, which is generally thinner in the middle portion of the 

 areolae, does really occasionally become highly silicified, and may 

 support particles of granules of highly refractive silica placed over 

 the so-called "eye-spots," in which case the cavities must be hermeti- 

 cally sealed on both sides to all but osmotic influences. 



(e.) That the lower closing membrane of the areolae frequently 

 carries various designs, the nature of which, on account of their 

 minuteness, has not yet been well established, but which must depend 

 upon structure, as no diffraction images produced by any organization 

 lying at a lower level can be the cause of them, as no such lower 

 organization exists below this bottom, or closing internal diaphragm. 



(/.) That the thin upper membrane of the areola} forms the exten- 

 sion of the edges of the so-called " nail-headed " bars which form tho 

 limiting walls of the areolae as figured by Otto Miiller, Flogel, Prinz, 

 and Van Ermengem. 



(g.) That the cavities in the valves are bounded by walls of solid 

 silica. That these walls often extend beyond, above, or below the 

 closing membranes of the areolae, and that they freqtiently run into 

 points or spines of various shapes and lengths, which project beyond 

 the valve between the areolae. 



(7t.) That the median slit or fissure, which is observed to run 

 through the rachis, or thickened median line of most of the Naviculse, 

 is also closed top and bottom by a very thin organic slightly silicified 

 membrane in recent normal valves. He believes, however, that 

 minute apertures may exist in these narrow closing membranes in tho 

 neighbourhood of the central and of the terminal nodules ; but this is 

 a subject requiring further elucidation. 



(i.) That the so-called " secondary" or internal valves — " Eegene- 

 rationshiille " — of some diatoms do not exist in the very young valves, 

 a fact which gives us the reason why the frustules which are formed 

 of an old and of a younger valve, generally split up into an odd num- 

 ber of secondary valves, either three or five. It is his belief that the 

 young secondary valves are always perforate at first, but as they 

 grow older successive depositions of silica generally take place 

 which end by obliterating the orifices, and in some cases fill these 

 quite up by dense and projecting masses of silica of a higher refrac- 

 tive index than the substance proper of the surrounding shell, so as to 

 appear as red or pink coloured granules on a greenish ground under 

 the best immersion lenses. 



(&.) That the connective zones or bands of some genera, such as 

 Isthmia, are really and truly perforate. 



(Z.) That the so-called "areola?," "beads," "pores," "orifices," 

 " granular projections," " depressions," " hexagons," " moniliform 



Ser. 2.— Vol. VI. 3 x 



