I9I5.] N. ANNANDALE: IJndtan Boring Sponges. 17 
distinct genera are included under the name. The genus as at 
present constituted is remarkable for the great diversity of its 
microscleres, which always include some form of amphiaster, as 
well as, in many cases, degenerate forms of euasters. True spiras- 
ters seem to be invariably absent. 
The typical spicule is characteristic. It consists of a relative- 
ly stout cylindrical stem, as a rule quite straight, and of two cir- 
cles of horizontal branches, which surround the stem at or near its 
extremities. The stem is quite smooth. In most cases the lateral 
branches are very short and greatly inflated at their tips, so that 
they have actually the form of subspherical bosses or prominences. 
They are never numerous, four to six being the normal number in 
each ring. In the more highly developed forms the prominences 
are covered with short spines, and the extremities of the stem are 
inflated and spiny also. Both the terminal and the lateral promi- 
nences may, however, be greatly reduced and take the form of 
smooth rounded or conical projections. 
Another form of amphiaster that is often, though not invari- 
ably, present also consists of a smooth cylindrical stem surrounded 
at the ends by a ring of horizontal branches. Both the stem (as 
a rule) and the branches (always) are, however, more slender and 
the latter are much produced. The extremities of neither are 
regularly spiny, but each branch terminates either in a minute 
inflation or in several small hook-like spines. 
A third form of microscleres that often occurs has been shown 
to be a degenerate oxyaster, although in its common form it has 
little resemblance to that type of spicule. As a rule it consists 
merely of two slender, more or less strongly curved spines attached 
to a minute centrum and having the appearance of the horns ot 
some Kuminant attached to a fragment of the skull, or that of a 
sea-gull in flight as seen from a distance, or rather as convention- 
ally represented in pictures. Occasionally more than two spines 
are present, and the spicule may assume a star-like form. Other 
microscleres, which resembles toxa but probably have the same 
origin, also occur in some species. 
Yet another type of aster is often found. It has the form of 
a flat, spiny plate or a spiny cylinder and is referred to by Top- 
sent as a pseudosteraster. I have not come across this form of 
spicule myself in the specimens I have examined. 
The macroscleres, if present, are either amphioxi or tylo- 
styles, but they are often absent. 
The distribution of Thoosa is essentially tropical, but several 
of the species are as yet recorded only from specimens of unknown 
history. They appear to occur mainly in shells of solid structure 
or teef-corals from shallow water, but one species described here 
(LT. tnvestigatoris, p. 18), was found in a thin Gastropod shell from 
a depth of over 700 fathoms. 
The following species have been recorded, or are here recorded 
for the first time, from the seas of British India and Ceylon :— 
