DoNKiN, on Marine Diatomacece. 
25 
differs as mucli from it as is possible for two nearly allied 
forms to do. 
Hab. Chibburn Mouthy Druridge Bay, abundant. This is 
tlie only locality in which it has occurred to me. In the 
Cresswell gathering, in which PL rectum was abundant, I 
could not find a single frustule of this species. 
10. PL arcuatum, n. sp. — V alve very pale-brown, straight, 
broadly lanceolate; extremities produced into two long, 
obtuse, strongly arcuate beaks, curved in opposite directions. 
Length from O'OOl/' to O'OOiG"; breadth about O'OOOS". 
Striae obscure. Median line straight, and terminating at the 
commencement of the extremities. 
The long, strongly arcuate, and somewhat obtuse, ex- 
tremities (resembling the bill of the curlew, Numenius ar- 
quata), and the short, wide body of the valve, distinguish 
the present species from PL macrum, to which, however, it 
is closely allied. It is, besides, much shorter than the latter 
species; never exceeding 0*005", which is about half its 
length, according to the measurement of Professor Smith, 
given in his ^ Synopsis.^ The extremities, also, are much 
longer in proportion to the valve than in PL macrum. 
Hab. Chibburn Mouth, Druridge Bay, and Cresswell, 
abundant. 
COCCONEIS. 
The species next to be described I have had some difficulty 
in referring to the present genus, in consequence of its frus- 
tules being free, and not adherent to the larger Algae ; a cha- 
racter which has been insisted upon by Ehrnberg, in the 
establishment of this genus, and adhered to by Professor 
Smith in his ^ Synopsis.^ The frustule in this species is also 
remarkable in not having the median line central. But in 
certain other respects it appears to me to be a true Cocconeis. 
I have therefore classed it as such, and have done so under 
the impression that, as our knowledge of the Diatomacese 
increases, it will be found necessary to extend the basis of 
distinction on which many of the present genera have been 
founded, in too limited and arbitrary a manner, in order that 
they may thus be made to embrace a much larger number 
of species, and thereby prevent the formation of new genera 
for the purpose of including every new form which may 
present certain structural peculiarities apparently anomalous. 
11. C. excentrica, n. sp. — Frustules free. Valve disciform, 
convex near the margin. Median line not central, with 
terminal nodules not reaching to the margin. Striae con- 
vergent, widely punctate, punctae closer and more conspicuous 
