WALKER-ARNOTT, ON RHABDONEMA. 89 
Doubts may arise whether what I have described as costz 
on the valve, are not strongly developed septa projected by 
the adjacent annulus and seen through the valve; and this 
hypothesis would increase the affinity of the first and third 
species to the second one. But a careful examination of the 
first species to be described, where observations can be more 
easily made than on the other two, leads to a contrary 
opinion: Ist, the striz are never seen on the valve except 
between the costz; on the lower valve they terminate where 
the costz terminate: 2d, I have not been able to detach a 
single valve so as to exhibit striz without the cost, or septa 
without striez. The two formations are thus dependent on 
each other, and the one is indissolubly united to the other; 
whereas in Rhabdonema and Grammatophora the septa can 
be separated, leaving the striated valve by itself. 
In all the three species the valve varies considerably in 
form; so that had I not obtained them in a tolerably 
separate state, I should have been puzzled whether to 
combine all imto one, or to constitute of them many more 
species than I have proposed. Indeed, when species of the 
same genus are much mixed and only known from deposits 
or dredgings, it is quite impossible to draw any satisfactory 
conclusions. 
Perhaps it might have been sufficient to unite the species 
I have to notice to Rhabdonema ; but the valves are not only 
furnished with costz, but these costz are differently ar- 
ranged on the two valves; whereas in all true species of 
Rhabdonema, the two valves are precisely similar to each 
other, and without coste. If the two genera be combined, 
the character must be enlarged, and then it will be no easy 
matter to exclude by it other genera which all agree are dis- 
tinct. I have therefore thought it preferable to separate the 
new one under the name of 
EUPLEURIA. 
Filaments compressed or arcuate, continuous, attached. 
Frustules annulate, indefinite; annuli plane, cellulete or striate 
on their circumference; septa opposite or alternate, rudi- 
mentary or perforate. Valves ovate, elliptical, or arcuate, 
with one median and several lateral costs; inferior with the 
coste and striz disappearing below the extremities of the 
valve, superior with them reaching to the extremity: striz 
moniliform, oblique. 
1. Eu. pulchella ; annuli as broad as the flat valves, cellu- 
late ; septa all rudimentary. 
Hab. New Zealand, on Polyzonia Harveyana. 
