English Soils : an Important Factor in Soil Biology . 69 
in the figure. The puncta were of variable size according to their position, 
the one terminating each striation near the raphe being very small and 
circular, while most of the others were larger and frequently oval in shape. 
The valve was 10 /x broad and about 25^ long, with 17 striations in 10 \x. 
9. Navicitla Pnpula , Kuetz. (Text-fig. 9 , 24 and 25). 
This diatom, as observed in the cultures of eleven soils, showed a 
considerable tendency to variation in shape. The valves were elongated and 
usually swollen opposite the median nodule, with rounded apices (Text-fig. 9 , 
25) ; but sometimes they were narrowly elliptical without any enlargement 
in the centre (Text-fig. 9 , 24). There was a straight hyaline zone surround- 
ing the raphe and swelling out abruptly opposite the median nodule to form 
a false stauros. The striations were radiating, and varied in number from 
iq. 10. 2/. 23. X4. 25 26. 27. 
Text-fig. 9. Soil diatoms II. 1-7, Navicula tmitica , Kuetz. formae, 5 possibly N Kotschyi , 
Grun. ; 8, N. mutica , var. quinquenodis , Del. ; 9-13, N. Atonius, Naeg. ; 14-17? N. content a, Grun., 
var. biceps , Del. ; 18-21, N. hyalina , Donk., var. minima, n„ var. (transverse striations doubtful) ; 
22 and 23, N. exilissima , Grun. ; 24 and 25, N. Pnpula , Kuetz. and formae; 26-8, P[. Pupula t 
var. undulata, n. var. Figs. 1 — 1 7 and 22-8, x 1435; figs. 18-21, x 1 7 1 5 - 
20 to 24 in 10 fx. The terminal nodules were enlarged sideways, but not to 
the extent that is figured in Cleve and Grunow’s ‘ Die arctischen Diatomeen 
Taf, II, Fig. 53 ; the valves were about 19 to 21 /x long and 4 to 4*5 broad, 
var. undulata , n. var. (Text-fig. 9 , 26-28). 
In two of the soils containing N. Pnpula , Kuetz., a second form was 
observed in considerable quantity which differed from the latter in its some- 
what larger size, 23 to 25 /x long by 4 to 5 /x broad, and in the shape of the 
valves. These were oblong with obtuse rounded ends and undulating 
