314 Proceedings of the Royal Irish Academy. 



FAMILY YI. STEIATELLE^, Kiitz. 



Fnistnles precisely as in the immediately preceding family, and 

 distinguished by the possession of internal diaphragms, which, spring- 

 ing from the connecting membrane, are interposed between the two 

 opposite valves. 



This family, since it was constituted by Kiitzing, has been adopted 

 by successive authors, but with some divergence respecting the species 

 comprehended within its limits. As here defined, it embraces not 

 only the genera included in Kiitzing' s family Striatellese, but also 

 those of the family Tabellaricaj, which, though generically distinct, 

 come properly within the same limits. The Genera Gephyria and 

 Eupluria, which were included by Ralf s, are here excluded from the 

 family ; because in whatever other respects they may agree, they stand 

 remote in consequenee of the unsymmetrical structure of the opposite 

 valves. And for a similar reason I exclude Podosphenia, which Hei- 

 berg placed in the family, the frustules being unsymmetrical both on 

 front and side view. 



Thus defined, the family is precisely equivalent to Grunow's sub- 

 gTOup of Diatomese, and stands out distinguished by two very decided 

 features, namely, the perfect .symmetry of the valves, and the inter- 

 position between them of diaphragms more or less numerous. 



The diaphragms constitute so important a feature in the family, 

 that some observations are needed to explain their nature and mode of 

 growth. As far as I know, Ralfs was the first who described these 

 organs with any tolerable approach to accui^acy. He says: — "The 

 appearance of longitudinal striae is in fact produced by silicious plates, 

 arising internally from the margins of the filament, and extend- 

 ing towards, but not reaching, the centre. The interior is thus 

 divided into chambers opening into a central space. When viewed 

 laterally, this central space resembles a canal, especially as the inner 

 edge of each plate has a concave outline." Ralfs, in Pritch., p. 803. 

 If there be anything vague in this description, it is greatly elucidated 

 by the observations of Wm. Smith, as well as by the figures of these 

 diaj)hragms in the case of Ehabdonema, Tetracyclus, and Tabellaria, 

 B. D., Yol. ii., pp. 32-34, PL xxxviii., fig. 306 h and 305t, PL xxxix., 

 fig. 308 h and h', PL xliii., fig. 31 Of and 31 Tf. 



In these cases, the diaphragms may be regarded as compressed 

 rings corresponding externally with the outline of the valves ; but 

 Heiberg has called attention to the fact that, in the case of Striatella, 

 the diaphragms are somewhat differently constructed. " Smith's repre- 

 sentation of the structure of this species is in the main correct, but he 

 has misapprehended the form of the diaphragms, considering them to 

 be closed rings (as his figure 307 h, as well as the descriptive term, 

 " Annuli" indicate), whereas in reality they are open at one end. The 

 form of them would naturally be best seen by preparing them out sepa- 



