On Erosion of the Surface of Glass, &e. By Dr. W. M. Ord. 767 



amination a piece of smooth shell — the internal surface of a 

 pinna — over which a colony of Lepralia inmctata has heen spread. 

 in this genus, as JMr. Busk has demonstrated to me, the ectocyst of 

 the under or attached surface of the coenoecium is not continuous, 

 but leaves oval spaces corresponding to the middle portions of 

 polypides, through which the soft tissues of the polypide come into 

 direct contact with the surface of the shell. When the coenoecium is 

 detached from the shell, its under surface is therefore seen to he 

 regularly fenestrated. And in the specimen of which I am speak- 

 ing, the surface of the shell from which the coenoecium has been 

 removed presents regular markings exactly corresponding to the 

 fenestra. These markings turn out, on microscopical examination, 

 to be depressions in the surface of the shell, produced by erosion. 

 It can be plainly seen that their floor is totally different from the 

 surface proper, both in colour and texture. The surface of the 

 shell is smooth, and reddish in colour ; the excavations present a 

 granular white floor. In another specimen, stained while the 

 animal matter was yet in its place, the points which would after 

 removal of the coenoecium have been occupied by the shallow pits, 

 have attached to every one a strongly adherent piece of dried stained 

 animal matter. Mr. Busk's observation is in fact an illustration of 

 my experiments, which shows what was sought better than did the 

 experiments themselves. Through the windows in the framework 

 of the coenoecium the organic matter of the polypides has, so to 

 speak, stencilled the structure upon the surface of the shell, as I 

 had tried to engrave letters on the surface of the glass. I cannot 

 resist the temptation of expressing the gratitude which I feel to so 

 great an authority as Mr. Busk for his kindness in giving attention 

 to my letter in ' Nature,' and in letting me have the opportunity of 

 seeing his specimens under his own demonstration. 



Another illustration of this kind of action was, as I think, 

 adduced by Professor Charles Stewart in the admirable course of 

 lectures on the Hydrozoa delivered by him recently at the Koyal 

 College of Surgeons. Professor Stewart showed dried specimens of 

 a species of Hydractinia investing shells of Gasteropods. The 

 shell had been completely covered by tlie fleshy expansion of the 

 Hydractinia, and had been deprived thereby of all its earthy 

 material. 



I have long believed that the action of the little sponges which 

 bore into molluscous shells, particularly those of Lamellibranchs, 

 was molecular. It is of course conceivable that the action might be 

 chemical ; that an acid might be excreted having the power of 

 dissolving carbonate of lime. It is also conceivable that the action 

 might be mechanical. The boring sponges contain siliceous spicules, 

 which, moved by the contractions ot the protoplasmic material, 

 might grind away the softer shell matter. I have tested fresh sec- 



