GREVILLE, ON DIATOMACE^. 9 



features are immediately appreciated by the eye. C. puncta- 

 tissima is more uniform, and considerably larger; but the 

 best mark is the conspicuous peculiarity of the median line, 

 which in C. placentula is simple. 



2. Cocconeis crebrestriata, Grev. Valve elliptical, delicately, 

 closely, and uniformly punctato-striate ; striee concentric 

 with the extremities ; median line straight, simple. Length 

 offrustule 0-0022" to 00028"; breadth 0-0012" to 0-0014''. 

 Stria3 30 in 0-001". (PL III, fig. 2.) 



A well- characterised species. The striae are numerous and 

 closely arranged, with a faint appearance, requiring careful 

 adjustment in order to render them distinct. Under higher 

 powers the sculpttu'c of the striae exactly resembles ordinary 

 cellular tissue, which under inferior powers causes the uni- 

 form oval-punctate appearance. Occasionally a border of the 

 valve is indicated by a faint line, as seen in the figure, but 

 this is not always apparent. 



3. Cocconeis inconspicua, Grev. Valve nearly circular, 

 border broad, rather strongly striated ; disc diaphanous ; 

 striae concentric with the extremities, faint, obscui'e in the 

 centre. Diameter of frustule O'OOll". Striae at margin 22 in 

 0-001". (PL III, fig. 3.) 



This is a most delicate form, so transparent as to be very 

 easily overlooked. The border being the most strongly 

 marked, first catches the eye, appearing like a mere striated 

 ring, until the median line and nodule be brought into focus. 

 The striae of the disc are perceptible for about a third of the 

 space between the border and the median line, when they 

 gradually become quite obscure. 



4. Campylodiscus fenestratus, Grev. Valve nearly circular, 

 the broad prominent border composed of a series of naiTow 

 cells; disc with four lattice-like sculptures formed by 3 — 4 

 bars crossing each other at right angles. Diameter of frus- 

 tule 0-0023". (PL III, fig. 4.) 



No drawing can do justice to the exceeding beauty of this 

 Diatom. The broad and prominent border is equal to about 

 a fifth part of the whole diameter, and is composed of narrow 

 parallel cells, which, at first sight, have the appearance of a 

 double series, a deception arising from an undulation in the 

 substance of the valve. Radiating striae appear to pass from 

 the border for a short distance towards the centre of the disc, 

 which is occupied by four remarkable sculptures, exactly 

 resembling square windows in miniature, the bars sharp and 

 slender, and the panes actually appearing as if they trans- 

 mitted light. The windows are not perfectly symmetrical, 

 as some of the rows of panes are larger than others ; never- 



