and on the removal of portions of their Shells. 241 



tained a desiccated animal of this borer adhering to the base of 

 an excavation, and on placing the foot and edges of the mantle 

 in the compressor, large, robust crystalline bodies (figs. 3, 4) 

 were distinctly visible, arranged in irregular groups around the 

 margin apparently in the mantle : similar bodies were also dis- 

 tributed over the foot, but without the slightest tendency to 

 order. They are mostly five- and six-sided, are thick, and have 

 an elevated point in the centre. When crushed between glass 

 they are liable to fracture in a radiating manner (fig. 5), as if 

 from the influence of the pressure on the central point. 



It is difficult to say what these crystalline bodies are composed 

 of, though there can be little doubt that they are modified epi- 

 thelium scales, from which they differ chiefly in being very ro- 

 bust, highly refractive and brilliantly crystalline. The difference 

 between these and ordinary epithelium scales will be at once 

 recognized, if a little of the surface of the lower portion of the 

 siphonal tube of Pholas be examined in the compressor. It ap- 

 pears also that, like the scales of epithelium, these bodies are con- 

 stantly being shed. On testing the scourings taken from the 

 bottom of the burrow of Saxicava, as before -mentioned, they 

 were found to contain a vast number of these bodies exactly cor- 

 responding with those of the mantle ; and on examining the 

 sediment adhering to the shell of Gastrochana, the residuum 

 after the action of the acid had ceased was almost entirely com- 

 posed of them. The reduced wood taken out of Teredo also con- 

 tains brilliant crystalline bodies resembling those in the foot 

 and mantle. Whether these bodies, however, are epithelium 

 scales or not, we see in this deciduous character the means of 

 keeping the rubbing surface in an efficient state. With pressure 

 these bodies frequently break into sharp angular fragments. 

 Acetic acid has no effect on them ; and in Saxicava strong nitric 

 acid produces no change even after several days' immersion. If 

 allowed to remain sufficiently long in this acid the imbedding 

 tissue is destroyed, and the crystalline bodies, not in any respect 

 altered, are left as a sedimentary residuum. Those of Pholas 

 and Teredo, however, appear to be ultimately acted on by this 

 acid, though they resist its power for several hours, and are never 

 totally destroyed by it. They become attenuated and brittle, but 

 retain much of their brilliancy and sharp angular appearance ; 

 and even some of them, especially in Pholas, are scarcely at all 

 altered after having been subjected to the action of this power- 

 fully corrosive acid for many daye. From these facts it perhaps 

 may be inferred, that these crystalline bodies are either entirely 

 composed of silex, or are a combination of it with animal matter. 

 These experiments certainly do not prove this ; but when their 

 results are taken in connexion with the crystalline appearance of 



Ann. $ Mag. N. Hist. Ser. 2. Vol. ii. 17 



