Mr. J. S. Bowerbank on the Siliceous Bodies of the Chalk. 255 



of the shell were always included in the empty space. I can only 

 say that I shall have much pleasure in showing him, at any time, 

 nine such specimens in one drawer of my collection. 



With regard to the fact that the shell of an Echinite should be 

 partially or wholly filled with silex, and then attached by some part 

 of its surface unconnected with the great orifices to a mass of flint, 

 it is in no respect unnatural among recent sponges. Two sepa- 

 rate individuals are often based upon the same stone or shell, and 

 if they grow sufficiently large to touch each other, they unite or- 

 ganically and form one sponge ; but if they be not of the same 

 species, they will grow over or envelope each other, but never 

 unite organically. I have several such specimens from Algoa Bay 

 and from Wollongong near Sydney, and of the latter description 

 I have one specimen which is composed of three species. Some- 

 times an individual of a different species will be developed upon 

 the very summit of another sponge, and both live and thrive 

 under these circumstances ; such a specimen I have from Wol- 

 longong. I have also from the West Indies a Verongia seven or 

 eight inches long so completely enveloped by a large fistulose 

 spongia that not more than about one inch of its length is ex- 

 posed, and yet both species were alive when taken from the sea. 

 Is it unnatural then, that among the Wiltshire flints we should 

 find one, two or three species of sponge included within another 

 parasitical and casing sponge, and that the included ones should 

 not be united to the enveloping one ? On the contrary, it is what 

 we see is the habit of the Spongiadse of the present day, and 

 therefore exactly what we should expect to have been the case 

 with the fossil species. And again, with regard to the filling of 

 the dead shells of Echinites and other hollow bodies with sponge. 

 It is true I cannot show the author an Echinus shell filled with 

 sponge, but thanks to my friend Mr. Pickering, who presented 

 me with the specimens, I can show him nine cases of the interiors 

 of bivalves of various genera, which have been, some wholly, while 

 others are only partially filled by the common sponge of com- 

 merce ; and what could scarcely have been expected, there is not 

 one of the casts in which the shell has been gaping, but eight of 

 them have had both valves closely shut, and in the ninth one very 

 nearly so, and in this the sponge extends by means of a thin 

 plate beyond the boundaries of the front of the valves of the 

 shell. In these cases, which afford beautiful casts of the interior of 

 the shell, and exhibit on their surfaces the impression of the mus- 

 cular attachments and striae of the valves, each cast has its cha- 

 racteristic enveloping membrane, and as the sponge has needed 

 no support, it has not attached itself to any portion of the inte- 

 rior sm-face of the shell. The author thinks it highly improbable 

 that teeth, wood and other extraneous bodies should be enveloped 



