BRYOZOA, WITH DESCRIPTIONS OF NEW SPECIES. 369 
on the upper surface ; the walls opaque, tough, and membranous, 
inclining to horny. Those of the latter, when examined through 
the microscope, exhibit a sort of dendritic structure; the divisions 
or branches passing in an irregular spiral direction round the 
tube, are flattened, and extensively anastomosing, form for the 
most part a dense tissue, nowhere more open than just to display 
the branched character. The walls of Plumatella do not in the 
least exhibit this structure. In Paludicella the polypidom, fig. 2, 
is likewise branched and tubular, but not carinated; it is mem- 
branous or horny, and becomes enlarged and contracted at cer- 
tain intervals, dividing the whole, as it were, into cells or com- 
partments, the external surface being smooth and very glossy. 
All these genera have the polypidom lined with a delicate 
membrane—the tunic, Pl. IV. figs. 4 6, b & 5 k, and Pl. V. 
fig. 1 6, which is attached only at certain points to the inner 
surface of the external tube or cell-wall. This, in Plumatella and 
Fredericella, becomes excessively delicate towards the orifice, 
where it apparently blends with the tunic. But in Paludicella 
the union at this point of the horny wall and tunic cannot be 
mistaken, though the blending is so gradual that it is impossible 
to say where one ends and the other begins. And when this 
polype is exserted, there is a delicate membranous cup, Pl. V- 
fig. 1 d, projecting upwards from the inner surface of the mouth 
of the cell. This cup is the homologue of the circle of setz sur- 
rounding the aperture of Bowerbankia and other marine genera. 
In Paludicella, the tunic is sprinkled with large nucleated cells, 
fig. 4 m, and at certain intervals bends abruptly inwards, figs. 1 
and 2 wu, r, dividing the polypidom into cells at the points indi- 
cated by the constrictions in the horny tube. Thus each polype 
is isolated, is contained in fact within a distinct membranous 
cell, the end-walls of which abut against the end-walls of the 
adjoining cells. The divisions are therefore double, and being 
of living membrane, and in contact, it is probable that all the 
inhabitants of the polypidom are in some degree connected in 
vital action. The end-walls are considerably thickened in the 
centre, forming a bulb or boss projecting into the cell. The 
polypes of Fredericella are not separated the one from the other, 
