254 Mr. J. S. Bowerbank on the Siliceous Bodies of the Chalk. 



in proportion to the crab as entirely to conceal it wben viewed 

 from above. There is no special preference on the part of the 

 crabs for any particular genus or species of sponge, but these 

 differ in almost every instance in my possession, and in one case 

 a single crab of the latter genus has three species fixed upon its 

 shell. The author continues : " It is assumed by this theory 

 that the sponges grew over the shells and other organic objects 

 which lay on the surface of the mud. But the observed facts 

 are wholly at variance with this assumption. ^^ I have before 

 my eyes at the moment of writing this, a sponge of the genus 

 Halichondria from Van Diemen^s Land of an oval form, seven 

 inches and a half long by five and a half wide, and not more 

 than two inches thick, which once grew spreading on the bottom 

 of the sea, and in the under surface of which sponge there are 

 more single valves and fragments of bivalves and univalves than 

 I can attempt to count with success ; and my friend Mr. Frederic 

 Catherwood, whose beautiful work on the Extinct Cities of Cen- 

 tral America has made him so favourably known to the public, 

 informed me that during a coasting voyage in a canoe, of about 

 100 miles, along the shore of South America, one of his chief 

 amusements was to lie with his head over the bows of the canoe 

 and feast his eyes with the splendid and variegated carpet of 

 sponges of all descriptions of form and colour, which almost co- 

 vered the bottom of the shallow sea over which he was voyaging. 

 But here again we need not transport ourselves to South Ame- 

 rica to illustrate this fact — the cave under St. Margaret's Island 

 at Tenby will suffice for our purpose. In this place I have found 

 seven or eight species of British sponges spreading over the sur- 

 face, and rendering the rocks beautiful with their tints of green, 

 orange, yellow, red, &c. The author then proceeds : " The Echi- 

 nites alone, extensively examined, afford conclusive evidence 

 against the sponge theory. These are very frequently indeed 

 found in the very centre of flints. They are sometimes found 

 with spines affixed, and therefore alive or with undecomposed 

 soft parts when entombed. The masses of flint to which they 

 are affixed are very frequently not attached to either of the large 

 orifices of the shell, but to some part of the sides, while the shell 

 is entirely filled with flint and both orifices closed. Mr. Bower- 

 bank states that, when the shell is not entirely filled with flint, 

 in 'the space thus unoccupied by the flint was always included one 

 or both of the large orifices of the shell.' I do not find this fact 

 in any degree borne out by my own observations.'' With regard 

 to the last sentence, I can only say, the author must have been 

 very unfortunate in his observations not to have found my asser- 

 tion " in any degree " borne out ; that in the cases where the shell 

 was only partially filled with flint, one or both of the large orifices 



