292 
in ■which the frustule can be viewed, together with the 
notched extremities of the valves, at once distinguish it from 
every other member of the genus. The central notch at each 
extremity is produced by the expansion of the carinse beyond 
the margin of the valve, so that a groove runs across the 
truncate extremities from the surface of one valve to that of 
the other; or, in other words, the groove across each extre- 
mity is continuous with the concave dorsum of the valves. 
I have had repeated opportunities of observing this both in 
living and mounted specimens. 
This form appears to be allied to S. didyma, Ktz. ( f Bac./ 
p. 60, t. iii, 67), and £. panduriformis, Raben. (‘ Siissw. 
Diat./ p. 29, t. iii, 8 and 9) ; but it is difficult to say what is 
meant by Kutzing’s and Rabenhorst’s figures. Kutzing de- 
scribes S. didyma as follows : “ Oblonga, utrinque truncata, 
medio sinuato-constricto, margine punctata/'’ and says it is a 
brackish water species from the Isle of Wangerooge. 
Rabenhorst more recently (‘ Diatom. Europ./ Leip., 1864, 
p. 51) has referred his S. panduriformis to this species; but, 
according to him, the M. V. is not panduriform, but oblong 
lanceolate, with truncate extremities ; while the valve on S.Y. 
is oblong panduriform, with truncated extremities ; the 
“ costae ” being fine, arising from marginal nodules ; his 
description is : E. minimis ; oblonga, utrinque truncata, 
panduriformis, a latere oblongo-lanceolata, utroque polo 
truncata. Costis parallelis gracilibus, e nodulis marginalibus 
ortis. v. v. Hab. in aquis submarinis litoralibus Germanise, 
Italiae, &c. Mr. Ralfs ( f Prit. Inf.,’ p. 794) thinks S. didyma 
is doubtfully a Surirella. It is, however, a submarine species, 
according to Kutzing and Rabenhorst; while the present 
species was discovered in the sandy bottom of a pool, and 
floating on the surface of a clear mountain stream several 
miles distant from the sea. 
2. Surirella subalpina , Donkin. 
Frustule on F. Y. broad, cuneate ; extremities truncated ; 
alae conspicuous. Valve ovate, elongated ; plicae transverse, 
slightly inclined near the extremities becoming incon- 
spicuous a short distance within the carinae, alternately 
costate on the inner surface of the valve. 
This species differs from S. splendida and other allied 
species in the alternate or iuner plicae being narrow and 
strongly costate ; in this respect it bears a close resemblance 
to S. limosa, Bailey. (‘ Quart. Jour. Micr. Sci./ Yol. VII, 
p. 179, PI. IX, 5.) 
Hab. Fresh water. Discovered in abundance in a clear 
