O'Meara — Report on the Irish Diatomacece. 315 



rately. One can, however, satisfy himself that the diaphragms are 

 open at one end, by obserring a perfect frustule in transverse view, 

 when one diaphragm is seen from the open, and the other from the 

 closed, side." — DeDanskeDiat., p. 72. In the case of the diaphragms 

 of Striatella, it may be noticed that the silicious plate is thicker at 

 one end, and becomes gradually thinner as it recedes from it. This 

 fact may serve to illustrate the mode of growth in the diaphragms in 

 other species, as well as in Striatella. Springing from opposite ends, 

 the two contiguous diaphragms in some cases appear much thicker at 

 the starting point, and as they proceed parallel to each other become 

 thinner and thinner as they approach the opposite end of the frustule. 

 The strong lines which appear as costse on the front view may seem to 

 mark the extreme length to which the diaphragms project into the cell ; 

 but the compressed rings are, except in the case of Striatella, com- 

 plete, and can by proper focusing be traced through their entire 

 coxirse. 



Genus I. Geammaiophoea, (Ehr.) 



Prustules attached, united in zig-zag filaments ; diaphragms two in 

 number ; open in the centre, and equally developed at both sides of 

 the same. Valves narrow, elliptical ; sometimes slightly expanded in 

 the middle ; striae obvious, and appearing on front view as a narrow 

 striate border. 



Although the species of this genus are by the experienced eye 

 easily discriminated, it is not easy to describe their characteristics 

 in words so as satisfactorily to obviate confusion with other forms 

 belonging to the same family, especially with Tabellaria, which they 

 resemble, not only in the mode of growth in zig-zag filaments, but 

 also in the general formation of the diaphragms, which are equally 

 developed on both sides of the central portion. Whether we view the 

 frustules on the front or side views, they may be distinguished by the 

 following characters : — In Grammatophora the valves are sometimes 

 slightly expanded in the middle, but in no case so much so as in Tabel- 

 laria ; the striae, too, are ever noticeable, which is not the case with 

 the' last-named genus. And on the front view the narrow margin of 

 strise noticeable in the species of Grammatophora are never to be seen 

 in those of Tabellaria. 



GrammatoiyTxora marina, (Lyngb.) Marine. 



Frustules on front view regularly quadrangular ; on side view 

 naiTow, elliptical ; strise obvious ; diaphragms curved near the ends, 

 and thence running in a straight line towards the middle. (PI. 29, 

 fig. 1.) 



Kiitz. Eac, p. 128, T. xvii., fig. 24, who regards the species as 

 identical with Diatoma marina, Lyngb. Wm. Sm., B. D., Vol. ii., 



E.I. A. PROC, 8ER. II., VOL, 11., SCIENCE. 2 P 



