TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 9 1 



intact. The openings in the internal plate, if there is one, as may be 

 the case in the Coscinodiscus, are in all cases closed in the living 

 diatom by the application of a cellular membrane. 



2. The walls of the opening may be upright, but most frequently 

 the opening tapers slightly at least towards the top, thus producing, 

 with the concurrence of the superposed membrane, a species of little 

 dome. It is this arrangement which causes the illusion of a bead. 



3. The openings of the grating assume an hexagonal form when 

 they are arranged in alternate rows. If, on the contrary, they are 

 placed in perpendicular rows they are square or extended. 



4. The walls of adjacent alveoles may be absent ; there is then a 

 confluence of these alveoles into a kind of tube or pipe ; such is 

 produced in the case of Finnularia (sub-genus of Naviculd) when 

 the grating exhibits a series of parallel tubes, which have been named 

 by some authors costca or canaliculi. 



The hexagonal form, which is so frequent in nature, appears to be 

 the typical form of the openings in the grating, and when the valve 

 is broad it is most frequently present without the costas of consolida- 

 tion and should offer assistance to exterior agents. Even in forms with 

 square openings, deviations and returns to the hexagonal type form are 

 very frequently observed on certain portions of the valve. 



These, then, are the principal features in the structure of the valves 

 of diatoms, so far as we can ascertain with such means as science is 

 able at the present time to place at our disposal. But the general 

 structure may appear complicated either by the presence of internal 

 secondary valves (Regenerations-hulle) or by deposits of silica produc- 

 ing in one place spines, granules, etc., in another place internal partitions 

 either true or false, in a third place costas, etc. 



All these deposits are only secondary productions of silica which do 

 not modify, in the slightest, the structure of the valve in its primary 

 features. 



In describing the valve, it is said to be alveolate or of cellular 

 structure when the alveoles are large, and appear polygonal ; it is called 

 moniliform or punctate when the alveoles are comparatively small. We 

 have already mentioned these terms at the beginning of this paragraph. 



C. Variation in the Striation. 



Former diatomists believed that the number of striae that could be 

 counted on a given space of the valve was fixed and invariable in 

 each species. This was asserted by Ehrenberg, amongst others, in 1835, 



