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 in 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 

 cabinet 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, differing but little in internal structure from 

 Halichondria. I have examined with much care the papillse 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 distinct 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 in its contractile power, — a quality surely of great im- 

 portance, raising Cliona in the scale of creation high above the 

 sponges in genei*al. 



From this striking character 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 alUed 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 individually so ; and they 

 attack inorganic as well as organic 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 re2;ions. 



