28 
DoNKiN^ 071 Marine Diatomacece. 
when tlie filament is exposed to the light for a short time. 
Is this singular movement^, with Avhich the present species is 
endowed^ not a vital phenomenon and independent of physi- 
cal causes for its existence ? 
6. When the moving extremity becomes entangled in any 
kind of substance intercepting its course, the opposite or 
stationary extremity commences to move, and continues to 
do so until the entangled extremity is set free; sometimes, 
in such instances, a frustule in the centre remains fixed, a 
movement of each half of the filament in opposite directions, 
on either side of it, taking place. But all these irregularities 
cease as soon as the impediment has been got rid of. 
These facts lead to the conclusion that the present species 
is a true Bacillaria, although apparently somewhat anoma- 
lous in the structure of its frustule. The gliding movement 
of one frustule over the contiguous one is the same as is 
observed in B. paradoxa. But it difiers from this latter 
species in this essential particular, that the ivhole of its fila- 
ment moves on one side of a terminal frustule which is stationary; 
while in B, paradoxa^ each half of the filament moves in 
opposite directions on either side of a central stationary 
frustule. 
Whether the filament is at first attached, and afterwards 
free, as in B. paradoxa, I cannot positively decide, although 
I believe it to be free, owing to its only occurring in the 
shallow furrows on the beach, where there is not a single 
vestige of vegetable life, except the free species of diatoms 
with which it is mixed. 
I think there can be little doubt that the form found by 
Professor Gregory in the Glenshira sand, and described as 
Nitzschia socialis, of which a group of frustules are figured 
Trans. Micr. Soc.,^ vol. v, PI. I, fig. 45) is another member 
of the same genus. This species,^^ he observes {op. cit. 
p. 80), is remarkable from its occurring in the prepared 
material, after boiling with acids, in groups of six, eight, ten, 
or twelve, or more, without any apparent connection between 
them." Groups such as these, of the present species, are 
common on slides mounted from gatherings in which it 
occurs. 
Amphiprora. 
13. Apr. duplex, n. sp. — F. V. rectangular, broad, rounded 
at the extremities, and very deeply constricted in the middle. 
Marginal plates also much constricted, and rounded at the 
extremities. Hoop broad. Structure exceedingly hyaline. 
Valve not striated, and ¥/ithout punctse on either side of the 
