402 On the Mollmcom Animals 



In assuming that one or other of these means is the method 

 by which they introduce themselves into rocks, how shall we 

 account for the statement (if true) of Spellanzi, namely, that 

 he found them in basalts and lavas ? rocks so much harder 

 than their shelly covering, and not easily acted upon by 

 solvents. The argillo-ferro-calcite, so often perforated by 

 them in the river Medway, contains abundance of Pholadia; 

 yet the Pholas crispata, and others, have their shells seriously 

 worn by their action against the sides of the cavity of a stone 

 comparatively soft ; and, if such is the case in the old shells, 

 it must be seen that the difficulty of their introducing them- 

 selves when in a young and tender state is almost insurmount- 

 able. The attributing to the stone subsequent induration 

 does not remove the whole of the difficulty. Again, many 

 of the Saxicavae are found extremely distorted and eroded, 

 evidently by the same action. It has, farther, been advanced 

 by authors that some of these animals have been detected in 

 granitic and some of the harder rocks. Residing, as I do, 

 on a primitive formation, where the nature of the rocks forms 

 a bar to the introduction of any thing like a shell, numerous 

 proofs might be adduced to oppose this opinion. So far 

 from this case obtaining, I have never observed any shell 

 embedded even in the softer part of a decomposing stratum. 

 The ilfytilus incurvatus, so abundantly found on this rocky 

 coast, affords one striking proof of the hardness of its usual 

 nest, as the simple action of opening its valves produces the 

 most serious and effective destruction of its surface. The 

 A'rca Notf', which lodges in the clefts of rocks, has, in like 

 manner, its exterior markings sometimes completely obli- 

 terated by the same action. It should be remarked that the 

 incautious conchologist may mistake the incrusting Polypi 

 spread over granitic rocks, and not suspect his error. 



The adequacy of the chemical action of any juice, produced 

 by the animal, to perforate such rocks cannot be entertained. 

 It cannot be denied that shells attached to hard rocks may 

 put on the appearance of having encaged themselves, so as to 

 deceive even the experienced conchologist: an instance of 

 this has recently occurred, where a specimen was sent to me 

 in proof of it. This was a block of silex [flint], rolled and 

 incrusted with marine substances, containing in a cavity a 

 beautiful specimen of the A'rca N(W, attached by its delicate 

 byssus [thready cable, or, vulgarly, beard]. To this cavity 

 it had conformed its shell so exactly as apparently to produce 

 indubitable proof of its eroding or solvent powers; but a 

 more intimate examination soon removed any doubts on the 

 subject. The block in question was found to be an adven- 



