EEPOET ON THE THNICATA. 
261 
I am inclined to regard the possession of only three rows of stigmata as being of less 
weight than the other two important characters taken together. Therefore if I had to 
decide upon a species forming a thick massive colony and having Ascidiozooids in which 
the atrial aperture was placed far back upon the dorsal edge of the thorax and had no 
atrial langnet, I would refer it to the genus Didemnum, even if there were four rows of 
stio-mata in the branchial sac. 
O 
The three new species which follow may be readily distinguished from one another 
even by their external appearance, as follows : — 
Didemnum savignii is of a chocolate-brown colour, and has few spicules. 
Didemnum aurantiacuni is of an orange colour, and has many spicules. 
Didemnum {f) inerme is of a light grey colour, and has no spicules. 
It is a little doubtful whether the last one really belongs to this genus, but the other 
two are typical species of Didemnum. 
Didemnum savignii, n. sp. (PL XXXIV. figs. 1-5). 
The Colony is a thick incrusting mass of irregular shape. It is attached by the entire 
lower surface, which is somewhat concave. The edges are thick and projecting. The 
upper surface is rather convex ; it is even, and quite smooth. The colour is dark brown. 
The length is 5'5 cm,, the greatest breadth is 3 ‘5 cm., and the thickness is 5 mm. 
The Ascidiozooids are of fair size, and are abundant. They are arranged in irregularly 
rounded systems, and their anterior ends form rounded spots of a yellowish colour on the 
upper surface of the colony, and generally about 0‘5 mm. in diameter. Several common 
cloacal apertures are visible ; they are large and distinct. The bodies of the Ascidiozooids 
are about 2 mm. in antero-posterior length, and less than 1 mm. in greatest breadth ; 
they are divided into two regions, thorax and abdomen. 
The Test is firm and cartilaginous. It is of a dark greyish-brown colour, and is 
moderately transparent. The matrix is clear and homogeneous. It is densely crowded 
with bladder cells and small test cells. A few stellate calcareous spicules are present in 
some places near the upper surface, but they are by no means numerous. Pounded and 
elongated pigment cells of a brown colour are abundant. They are placed chiefly in the 
upper part of the colony. 
The Mantle is fairly strong and muscular. The muscle bands are mainly longi- 
tudinal, and are placed with considerable regularity. 
The Branchial Sac is moderately large, and is well developed. There are three rows 
of large and regularly arranged stigmata on each side. The transverse vessels are provided 
with muscle fibres. 
The Endostyle is very large and conspicuous. 
The Dorsal Lamina is represented by a few large languets. 
