12 GREVILLE, ON DIATOMACE^E. 



I am not aware of any descriljed species which approaches 

 the present minute form. The strise are slightly radiate, 

 and though quite evident at the margin, become gradually 

 faint. The line which runs on each side, parallel with the 

 median line, passes through the valve, as it were, to form the 

 produced extremities. 



9. Phurosigma compactum, Grev. Valve linear-lanceolate, 

 obtuse; flexure of the median line so great that it touches 

 the margin, about midway between the central and terminal 

 nodules, the curve following the same gradient to the ex- 

 tremity; striae obscure. Length of frustule 0'0035" to 

 0-00 i5"; breadth 0-0006" to 0-0008". (PI. Ill, fig. 9.) 



This Pleurosigma, of which I possess several specimens, is 

 well marked by the excessive flexvire of the median line, 

 which is greater than in any species known to me, Avhile 

 the flexure of the valve itself as it is presented to the eye is 

 so moderate, that a straight line drawn from the terminal 

 nodules through the central one, passes considerably within 

 the margin. From the point where the curve of the median 

 line touches the margin, the latter, to the furthest extremity, 

 is nearly straight, and is just perceived exterior to the curve 

 of the median line, as the latter approaches the nearest end. 



10. Mastogloia minuta, Grev. Valve elliptical-oval to ellip- 

 tical-oblong, conspicuously apicidate ; loculi 12 to 18; striae 

 very fine and close. Length of fi-ustule 00008" to 00010"; 

 breadth 0-0004". (PI. Ill, fig. 10.) 



Although I have seen numerous frustules of this minute 

 Diatom, I have been unable to obtain a front view. As a 

 species it is evidently allied to M. apiculata of Smith ; but it 

 differs in being scarcely half the size, and essentially in the 

 much larger loculi. It is also much more decidedly apicu- 

 late, being generally even strikingly rostrate. 



