1849.] BOWERBANK ON A SILICEOUS ZOOPHYTE. 321 



pol}^idom is not its natural character, but has been induced by the 

 exhaustion and subsequent death of the animal, having prevented 

 their complete withdrawal within the polypidom. 



Each of the mammillae is furnished with a number of smooth cylin- 

 drical tentacula averaging -^^^ of an inch in length ; their diameter 

 at the base is 333^3^ inch, and they decrease gradually thence to the 

 apex, terminating in a blunt point. No portion of the surface of 

 the polypidom is furnished with these organs excepting the mam- 

 millae. 



The tentacula present every appearance of perfectly flexible organs ; 

 no semblance of rigidity exists, but they are bent and curved in an 

 easy flowing maimer in every possible form and direction. 



The animal is similar in structure to Prof. Edward Forbes' s genus 

 Sarcodictyon, but it has not the regularity in the disposition of the 

 cells which exists in that genus. 



In the tuberculated surface of the polypidom it much resembles 

 that of Sarcochitum polyoum ; but the habit of the animal is exactly 

 that oi Alcyonidium parasiticiim of our own coast, specimens of which 

 I have frequently taken at Scarborough, surrounding the slender 

 stems of Sertularia and other zoophytes. 



Nearly the whole of the animal within the agate is in a beautiful 

 state of preservation, but there are a few spots which present evidence 

 of the commencement of decomposition, by the detachment of groups 

 of cells from the mass of the polypidom ; in these cases the remains 

 of the tentacles, as might be expected, are very rarely to be seen ; 

 and the disrupted mass is totally without the sponge fibre. 



The envelopment of a tooth, a bone, or of hard calcareous bodies 

 such as shells, afford no definite information regarding the time ne- 

 cessary to accomplish such an operation. The investment even of 

 such bodies as the rigid, endurable horny fibres of that tribe of sponges 

 which are usually to be observed imbedded in flints, cherts, and moss 

 agates, give also a considerable range of time to accomplish the fos- 

 silization ; but when we see such a soft and perishable substance as the 

 fleshy body of the living Alcyonidse, and such delicate organs as the ten- 

 tacula of the polyps, thus preserved with such evident appearances 

 of freshness and perfection, I own that it excites in me the greatest 

 astonishment that there should have been so rapid a deposit of sili- 

 ceous matter as must evidently have taken place, thus to entomb the 

 animal in such a condition as proves, that at the utmost but a few 

 days must have elapsed before it was so far incrusted as to completely 

 preserve the form and position of the animal, not by a sudden im- 

 mersion in supposititious siliceous paste impounding it instantly in its 

 full vigour, but after a slow and gradual decease ; for this condition 

 which 1 have described, of semi-protrusion of the tentacula, is that 

 with which every one acquainted with recent zoophytes in a living 

 condition, is so familiar as an indication of slow and undisturbed death 

 by exhaustion. In this condition of semi-protrusion I have seen the 

 animals of Alcyonium digitatmn, Alcyonidium parasiticum, Caryo- 

 phyllea Smithii, and numerous species of Sertularia and other zoo- 

 phytes, die, if allowed to do so, without interference ; but if touched 



