322 Mr. A. Hancock on the Excavating Powers of Sponges, 
chambers are so well preserved in the fossil shells of these for- 
mations that the species may be determined. 
A new generic form has also been discovered : it is closely re- 
lated to Cliona, and like it conceals itself in calcareous bodies. 
Two or three species have occurred : they and several of the more 
characteristic species of Cliona that have come under my notice 
will be described in the sequel of this communication : the others 
are reserved for some future occasion. 
It is now upwards of twenty years since Professor Grant’s 
paper on Cliona celata appeared in the ‘ Edinburgh New Philo- 
sophical Journal’; and in 1840 M. Duvernoy described im the 
‘ Revue Zoologique’ another species inhabiting the shell of Ostrea 
hippopus. These I believe are the only species hitherto known, 
though extensive traces of the genus are to be found in every 
= of shells, and probably on every shore of the British 
islands. 
Professor Grant believed Cliona to be polypiferous. Such be- 
lief, however, has not been confirmed by subsequent observations, 
which seem on the contrary to prove that this production is 
truly a sponge, differmg but little in internal structure from 
Halichondria. 1 have examined with much cave the papille of 
Cliona when just removed from the sea, but have not succeeded 
in detecting any polypes. The propriety, nevertheless, of retain- 
ing it as a distinet genus would appear evident ; for though it un- 
doubtedly possesses many characters in common with Halichon- 
dria, yet Cliona differs widely from it in its habits, and parti- — 
cularly m its contractile power,—a quality surely of great im- 
portance, raising Cliona in the scale of creation high above the 
sponges im general. 
From this strikig charaeter it perhaps might be inferred that 
Cliona, and likewise Thoosa, by which name I propose to desig- 
nate the new generic form, are closely related to Tethea, which 
is stated to be irritable ; and as the two former are both provided 
with siliceous bodies or granules on the surface, as will be after- 
wards shown, they would also appear to be allied to Geodia, the 
external covering of which is composed of siliceous globules. 
The numerical extent of the species of Cliona cannot at pre- 
sent be estimated. Those now recorded are the result of a very 
limited investigation. That they are very numerous is evinced 
by the fact, that from a single specimen of Tridacna gigas a dozen 
species at least have been obtained. They are not merely spe- 
cifically numerous, but are likewise mdividually so; and they 
attack inorganic as well as organie bodies. They appear to be 
pretty generally diffused over the surface of the globe, though 
most numerous in warm climates: none have been yet procured 
from the Polar regions. 
