whilst others do not.  The mouth of the unarmed cells in both species is more or less circular and plain, but in C. bilabiata, even in the unarmed cells, the mouth is occasionally distinctly bilabiate.  In C. pumicosa the avicularium is placed subapically on a solitary posterior obtuse mucro, but in C. bilabiata there are two such processes longer and more pointed, one in front and the other behind the mouth; the avicularium, as in the former case, being placed immediately below the apex of the posterior mucro.  The ovicells also differ very much.  In C. pumicosa this organ presents several rather large circular spots or perforations ? whilst in C. bilabiata it exhibits a scutiform or horseshoe-shaped area, marked with several transverse lines on each side of a middle longitudinal line.

Fam. 7.  GEMELLARIADAE.

Cells opposite, in pairs.

22.  DIDYMIA, n. gen.  Table 1 figure 6.

Cells joined side by side; opening large, oval; mouth subapical, central.  No avicularium.  Ovicells contained within a cell, which is central at each bifurcation.

1.  Didymia simplex, n. sp.  Table 1 figure 6.

Cells oblong, narrowed below, broad and truncate, with an angle externally above.  Back marked with transverse rugae.

Habitat:  Bass Strait, 45 fathoms.

A fine species, growing in loosely-branched phytoid fronds, to a height of several inches.  In some (dried) specimens the branches are a little incurved, but not in all.  The situation of the ovicell is peculiar.  It is contained within the upper part of a cell placed between, or rather in front of the pair, from which the two branches at a bifurcation take their origin.  The ovigerous cell differs widely in form from the others, being pyriform, and much attenuated below; and the orifice is below the middle.  The upper compartment, in which the ovicell or sac itself is lodged, appears to be separated from the lower by a transverse diaphragm.

23.  DIMETOPIA, n. gen.  Table 1 figures 7 to 9.

Cells joined back to back; the mouths of each alternate pair looking in the same direction, and at rightangles to the intermediate pair.

1.  D. spicata, n. sp.  Table 1 figure 9.

Cells infundibuliform.  Margin of opening much thickened, with six equidistant, elongated pointed spines.

Habitat:  Bass Strait, 45 fathoms.

White, transparent, forming thick tufts about 1 1/2 to 2 inches in height.  The same species also occurs in New Zealand.

2.  D. cornuta, n. sp.  Table 1 figures 7 and 8.

Cells suddenly contracted about the middle.  Opening oval, wide above; margins slightly thickened with a short thick conical horn on each side above, and a long projecting spine (rarely two) in front below.

Habitat:  Bass Strait, 45 fathoms.

Branches narrower than in the preceding species.  Colour yellowish.  Tufts loose; ovicell small in proportion to the size of the cells.  It is placed immediately above and behind the upper margin of the opening of the cell to which it belongs.