26 
DoNKiN, on Marine Diatomacece. 
near the margin, tlius forming a somewhat opaque and 
broad marginal band in the dry valve. Diameter from 0*001 " 
to 0-002". 
The eccentric position of the short median line seems to 
distinguish this species from every other member of the 
genus. The frustules are free, and have the power of 
moving. The endochrome is central, and of a pale- green 
colour, leaving the marginal portion of the valve with its 
radiate strise distinctly visible. The dry valve is colourless, 
and the central portion much more translucent than the 
marginal. In balsam the opaque appearance of the marginal 
band becomes obliterated. 
Hab. Linemouth, abundant ; Cresswell, plentiful. 
Bacillaria. 
12. B. cursoria, n. sp. — Frustules adherent, by means of 
some invisible connecting medium, into a filament, and hav- 
ing the power of moving, one along the opposed surface of 
the other. Form narrow, linear lanceolate, acute, with a 
central longitudinal line composed of thickly set, transverse 
punctse, extending from one extremity to the other. Struc- 
ture hyaline, not striated. Length from 0-0028" to 0-0034"; 
breadth about 0-0004". The only view which I have ever 
seen of this species is that which always exhibits the frustule 
with the central, longitudinal, and punctate line uppermost, 
as in fig. 12a. Whether this punctate line is situated on the 
middle of the S. V. and indicates a keel, or whether it ex- 
tends down the centre of the F. V., corresponding with the 
intervalvular space, I am not prepared positively to decide, 
although I incline strongly to the latter opinion. My reasons 
for this opinion are the following : 
First. Because a group of dry frustules preserving their 
natural, relative position, always presents each frustule to 
view with this line in the centre, and because such a group 
or filament always exhibits the F. V., and not the S. V., of 
each individual frustule. The same is seen in Si similar group 
or filament of B. paradoxa ; never the S. V. 
Secondly. Because in frustules which appear to be under- 
going the process of self- division, the punctate line is double, 
the distance between the two lines varying according to the 
degree to which the process has proceeded. (See fig. 12 6, 
PI. III.) 
This species is so abundant on the sands in some localities 
on this coast, that, in the clean gatherings from these, I have 
enjoyed frequent opportunities of observing the movement 
