a General History of the Marine Polyzoa. 5 
Oecium rounded, smooth and silvery, somewhat compressed, 
on the front an area inclosed by a raised line, which is 
minutely pitted over. 
Loc. Off Curtis Island, on Retepora &e. 
Membranipora radicifera, n. sp. 
(Pl. Il. figs. 6, 6 a, 60.) 
Zoecia very large, quincuncial, ovate, attached by means of 
numerous slender radical tubes given off from the dorsal sur- 
face; aperture occupying the whole front, with a membranous 
covering; margin rather thin and smooth; two short and 
stout spines at the top of the cell, and a little below on one 
side a single forked spine; opposite to it a very large sessile 
avicularium, placed on the margin and occupying a great 
proportion of the side, somewhat turned over towards the 
area, with a rather slender pointed mandible, directed upwards, 
the beak hooked at the extremity ; occasionally an avicu- 
larium on each side of the zocecium. Owcium (2). 
Loc. Bass’s Straits, spreading over soft mud. 
This is the most interesting form which occurs in the 
Warren collection. It is, 1 believe, the only known Mem- 
branipora in which attachment is effected by means of tubular 
fibres emitted from the dorsal surface of the zocecia. In 
other cases the members of this genus adhere directly by the 
base of the zoarium, which is closely soldered to the substance 
on which the polyzoon grows. But in M. radicifera each 
cell gives off a cluster of very long slender tubes, so that the 
inferior surface of the zoarium is completely villous (PI. II. 
fig.66). This structural modification points of course to some 
peculiarity of habitat; and accordingly we find that the 
Bass’s-Straits specimens had evidently been spreading over a 
soft mud filled with small particles of shell, stone, &c., into 
which the long rootlets had penetrated, thus holding the poly- 
zoon firmly to its place. Whether the structure is constant 
in the species as it now exists, or whether it is an exceptional 
adaptation to peculiar circumstances, | am not in a position to 
say. It exists in all the specimens I have examined; but 
they were all subject to the same conditions. 
We have instances of remarkable plasticity in the radical 
appendages of the Cellulariide; but in this case the very 
existence of a radical appendage is an anomaly, attachment 
being effected in this tribe, as 1 have said, by simple adhesion. 
Another structural peculiarity occurs in this species which 
deserves notice. The cells are not united in the usual way ; 
they are partially disjunct (Pl. II. fig. 6a). Hach one is 
