310 
THE VOYAGE OF H.M.S. CHALLENGE^. 
upper layer of the test, while it causes the colony as a whole to become firmer and more 
solid looking. 
In this genus I place von Drasche’s Diplosoma pseudoleptoclinum from the Adriatic, 
and a new species described below, which was obtained during the Challenger Expedition 
off the Arrou Islands. These two species differ in a number of points, including the 
following; — In Diplosomoides pseudoleptoclinum the colony is thinner, more incrusting, 
and decidedly firmer, the spicules are more numerous, and there is a group of spicules 
placed on each side of the thorax. In Diplosomoides molle, on the other hand, the 
colony is thicker, very slightly attached, and very soft and flexible, the spicules are com- 
paratively few, and there are none alongside the thorax. The branchial sac is also 
relatively larger than in Diplosomoides pseudoleptoclinum. 
Diplosomoides molle, n. sp. (PI. XLII. figs. 5-16). 
The Coloyiy is an irregularly rounded mass, attached slightly at various points. The 
surface is uneven but smooth. The colour is dull white with a greyish tinge. 
The length is nearly 4 cm., and the breadth is 2 ‘5 cm. The average thickness is 
about 5 mm. 
The Ascidiozooids are fairly large and very numerous. They are distributed 
evenly all over the surface of the colony, and are closely placed. Several common cloacal 
apertures are present. The bodies of the Ascidiozooids are from 1‘5 mm. to 2 mm. in 
antero-posterior length and about 0’5 mm. in greatest breadth. They are distinctly 
divided into two regions, thorax and abdomen. 
Die Test is exceedingly soft and flexible ; it is easily torn. It is of a greyish-w’-hite 
colour externally, and of a clear hyaline grey internally, and is transparent. The matrix is 
clear and structureless. In the superficial layer of the colony it contains great numbers 
of small spherical or stellate calcareous spicules, while in the deeper parts of the test no 
.spicules are present. Test cells are very abundant, they are mostly of small size and 
rounded form. No bladder cells are present. Vascular ectodermal appendages are 
frequently met with in the test. 
The Mantle is thin and transparent. The chief muscle bands run longitudinally. 
The Branchial Sac is large and well developed. There are four row's of fairly large 
stigmata, arranged with regularity. The transverse vessels are moderately wide, and are 
provided with muscle fibres, which are also continued into the longitudinal interstigmatic 
vessels. 
The Dorsal Lamina is represented by a series of long narrow languets. 
77ie Tentacles are well developed, and are arranged with regularity. There are sixteen, 
and they are of two sizes, placed alternately. 
The Dorsal Tubercle has a narrow slit-like aperture running antero -posteriorly. 
