EEPORT ON THE THNICATA. 
237 
the edges, when held to the light, is a very dark brown, but on the surface it looks quite 
black. 
The test is hard and tough. In thin sections it is transparent, but in mass is 
quite opaque. The structureless matrix is densely crowded with test cells. In some 
places they are so numerous as almost to touch one another (PI. XXXI. fig. 18, t.c.). 
These test cells are nearly aU spindle-shaped or stellate, with large central nuclei, and 
with the protoplasm so full of small dark granules that it might almost be called 
pigmented. In addition, however, there are larger cells of ovate or globular form which 
are much more deeply coloured and are true pigment corpuscles. The pigment granules 
are dark brown or nearly black in colour. 
Each longitudinal muscle band in the mantle has four to eight fibres (Pi. XXXI. 
fig. 19, m.h.), and these fibres are exceptionally broad. Some parts of the mantle are 
slightly pigmented (PI. XXXI. fig. 19, p.c.). The branchial sac is thick walled and 
opaque, and the stigmata are inconspicuous. 
The alimentary canal is moderately large. The stomach is rounded, and its wall is 
folded longitudinally. 
The post-abdomen is short, and in the Ascidiozooids examined the reproductive 
organs were in an undeveloped condition. 
Psammaplidium, n. gen. 
Colony incrusting, massive, or lo-bed. 
Systems inconspicuous. 
Ascidiozooids usually small, not much elongated, and not distinctly divided into 
regions. 
Test thick, and greatly strengthened by imbedded and incrusting sand-grains and 
other foreign bodies which form a great part of its bulk. 
Branchial Sac small, and not well developed. 
Post- Abdomen usually short. 
I have separated this group of species from the other Polyclinidae on account of the 
very abnormal condition of the test which they exhibit. This region of the colony in 
all of these species contains, in a more or less marked degree, sand and shell fragments, 
&c., and these foreign bodies constitute, in most cases, a very considerable part of the 
investing mass. This sand is not merely incrusting, but the grains are actually 
imbedded in and surrounded on all sides by the test substance. Evidently in this genus 
the test has the power, during its whole development, of taking up foreign particles in 
large numbers and of finally growing over them, so that they come to be placed in its 
interior. This power is of course seen, to a limited extent, in the region of the test 
which forms the base of attachment in any fixed colony, and in the branched hairs 
