464 Rev. T. Hincks on the 
slanting inwards to the base, truncate below (lozenge-shaped) ; 
area slightly depressed, smooth, margin distinct, subcrenu- 
late ; orifice semicircular, situated in the upper third of the 
area, lower lip arched. Avicularian cells in the line of the 
ordinary zccecia, which they resemble, but are shorter and 
very much broader (about twice the width), prominent 
above, almost the whole of the upper portion (more than a 
third of the length) occupied by a semicircular orifice, which 
is filled in by a stout mandibular plate of a very dark horn- 
colour, the edge black. Occial opening at the very top of 
the cell, and of much the same shape as the orifice. 
Loc. Houston Stewart Channel; Virago Sound. 
C. mandibulata bears a close resemblance in most respects 
to C. fistulosa, and is separated from it on the strength of the 
very marked differences in the avicularium, which is found 
to be the best criterion for distinguishing specific forms in 
this genus. The avicularium of the latter is (morphologically) 
a dwarfed cell, with the oral valve slightly modified. In the 
present species the avicularian cell is in some respects larger 
than the ordinary zocecium, from which it is distinguished 
chiefly by its great breadth, its prominence, and its ample, 
dark-coloured, semicircular mandible (or modified oral valve). 
It represents one of the earliest stages in the developmental 
history of this appendage*. 
It may be a question, perhaps, whether C. mandibulata 
should not be regarded asa “ form’”’ of C. fistulosa; but it 
has much the same kind of claim to specific rank as C. sin- 
uosa. After all, these systematic distinctions are only meant 
to mark the developmental steps. 
Family Membraniporide. 
Group a (fuvsrerp2). 
Fiustra, Linneeus. 
Flustra membranaceo-truncata, Smitt. 
Virago Sound, 8-15 fms. [North Sea, Arctic Seas, 
common. | 
Group 2. 
MeEmBRANIPORA, De Blainville. 
Membranipora unicornis, Fleming. 
Houston Stewart Channel, 8-20 fms. ; very fine. [Spitz- 
* We have a very similar form in Cellaria hirsuta, MacGillivray, and 
Membranipora longicornis, mihi. See ‘ History of British Marine Polyzoa,’ 
Introduction, p. lxviii, fig. xxx. 
