DoNKiN_, on Marine Diatomacece. 
19 
discovered^ that it becomes essentially necessary to establish 
an entirely new genus to which to refer then. This genns 
(to be characterised by the curve of the median line^ and by 
the structure of the valve) I have termed Toxonidea [rol^pv 
and Iheuj bow-shaped). 
Toxonidea. 
Frustules free; valves elongated_, convex, with two sides 
not symmetrical ; striated, striae oblique. Median or longi- 
tudinal line arcuate, with central and terminal nodules, the 
latter curving towards the same side of the valve. 
One distinguishing feature, then, of this new genus, in 
addition to the arcuate median line, being the oblique striation 
of the valve (probably due to cellular structure), found so 
well developed in one section of the genus Pleurosigma, it is 
evident that it bears a close natural affinity to this latter 
group, and that both are members of the same sub-family. 
That the arcuate curve of the median line, together with the 
want of symmetry observable in the opposite margins of the 
valve, which indeed is strongly arcuate on one side, in one of 
the forms, and slightly so in the other, to say nothing of the 
total absence of any sigmoid tendency, is a structural diffe- 
rence sufficient to warrant the separation of these two species 
in question from the genus Pleurosigma, is apparent from the 
fact, that to admit them into it would be equivalent to 
abolishing the most distinctive character on which that group 
has been founded by Professor Smith, who observes that 
" the sigmoid flexure of the valve, more or less present in all 
our native species, at once distinguishes this genus from its 
allies.^^ 
1. Tox. Gregoriana,"^ n. sp. — Valve straw-coloured, lanceo- 
late; extremities obtuse, and curved strongly towards one 
side of the valve. Median line, on each side of the central 
nodule, curved first towards one side of the valve, then, some 
distance from the extremities, gradually and more strongly 
towards the opposite, until it reaches the terminal nodules ; 
when viewed from one extremity to the other it has a most 
graceful appearance, resembling a representation of an un- 
bent Scythian bow. Length from O'OOS" to 0-009"; breadth 
from 0-001" to 0-0016". Stri^ oblique, fine, probably 50 in 
0-001".t 
* I have dedicated this species to Professor Gregory, my former and 
highly esteemed teacher, by whom our knowledge of British marine Diato- 
macese has been considerably enlarged. 
f 1 may here state that, in describing the strice of this, as well as of the 
following seven species, I have merely attempted to guess the number of 
