228 Mr. A. Hancock on the Boring of the Mollusca into Rocks, 



have also a delicate, wrinkled epidermis, which spreads over the 

 anterior spiny portions of the valves. It is sometimes imperfect, 

 but more or less of it may be always detected : in fine specimens 

 it even covers the spines. All the British species have it. I have 

 seen it very finely exhibited in specimens in Mr. Alder's cabinet ; 

 particularly on P. crispata, P. dactylus and P. papyracea. This 

 epidermis is so delicate that it is liable to be removed by the 

 washing-brush, and it is probably owing to this cause that it 

 has been so little noticed. Some years ago Mr. Gray pointed 

 out the existence of an epidermis on these shells. After this it 

 may seem unnecessary to say anything more on this branch of 

 the subject, for a stronger proof than that just noticed of the 

 inefficiency of the spines as rasping instruments can scarcely be 

 produced. But another fact has come under my notice, and as 

 it relates to P. Candida, the species which Mr. Osier describes as 

 boring with its valves, it should not, perhaps, be passed over in 

 silence. On the coast of Northumberland this species, old and 

 young, is generally incrusted with a fine dark sediment, which 

 frequently covers the whole of the shell, including the spines. 

 On the back of the valves, towards the umbones, it is very com- 

 monly removed by rubbing against the sides of the burrow, and 

 the shell, at this part, is often nearly worn through. It is, there- 

 fore, pretty certain that in these cases no rasping by the spines 

 in front could have taken place. 



It is also worthy of remark, that the Pholades commence to 

 bore immediately after leaving the parent. I have removed from 

 excavations the fry of P. crispata about one-fiftieth of an inch in 

 length, being scarcely more than the mere nucleus, of excessive 

 tenuity, brilliantly glossy, and as yet unprovided with spines, 

 except two or three on each valve in a state of growth. They 

 had nevertheless made for themselves regularly-formed excava- 

 tions. How ? — The advocates for the mechanical rasping of the 

 valves will find much difficulty to explain. 



The excavations at first are only one-sixtieth of an inch in 

 diameter ; but as the shell grows and sinks deeper into the sub- 

 stance the bore widens ; and as the increase is more below than 

 above, it is of a conical form with the apex upwards. What I 

 wish to draw the attention to here is, that the burrow does not 

 only deepen and increase in diameter below, but also that it 

 widens above the shell ; and that the orifice, in adult individuals, 

 is often augmented to one-fourth of an inch in diameter. Were 

 the shell the only boring instrument, this could not happen ; for 

 the channel above it incloses the siphonal tubes, and after the 

 shell has once passed through this part of the burrow it cannot 

 again return on account of its increased size. It is clear, there- 

 fore, that the soft fleshy wall of the siphonal tubes widens this- 

 part of the channel : and this would appear to establish the fact 



