and on the removal of portions of their Sheila. 227 



quate to the task assigned to them. The bottom of the excava- 

 tions of all the boring bivalves, — of the Pholades as well as the 

 TeredineSy — is regularly rounded or scooped out in a manner that 

 could scarcely result from the rasping of shells formed like those 

 of these genera : certainly not without extensive rotatory motion, 

 and for reasons before-mentioned it seems pretty clear that in 

 Teredo the power to rotate is very limited. In Pholas, too, it 

 would appear that there is not much rotatory action, as we shall 

 afterwards endeavour to show. Indeed, it seems quite impossible 

 to bring what is usually considered the rasping j)ortion of the 

 valves of P. crispata into contact with the bottom of the burrow ; 

 consequently in this species they can hardly be considered as 

 boring instruments, even supposing that the spines were capable 

 of rasping the softer sandstones, shale, chalk, wood, and lime- 

 stone. In Xylophaga we have a pretty good proof that the an- 

 terior portions of the valves do not come into contact witli the 

 bottom of the burrow. This genus has the habit and general 

 appearance of Teredo, but is distinguished from it chiefly by two 

 accessory valves, which are so placed as to preclude, apparently, 

 the possibility of this taking place. And in further proof that 

 the spines are not used for such a purpose we have but to examine 

 them : they are frequently quite sharp and perfect, bearing 

 neither scratch nor blemish of any kind. It may be urged, un- 

 doubtedly, that nearly all these animals penetrate only the softer 

 substances, such as chalk and shale ; but it must not be forgotten 

 that these bodies contain siliceous particles, and that P. striata, 

 and most of the Teredines, burrow in the hardest oak. On the 

 coast of Northumberland P. crispata is not unfrequently found 

 in shaly sandstone — a material quite capable of reducing a much 

 harder shell than it possesses. This species also occurs in the 

 softer limestones in the same locality and on the Durham coast, 

 and P. dactylus, P. parva and P. papyracea occur in the lias, 

 which is hard and compact. There can be no doubt, however, 

 that the softer substances alluded to above can reduce shell. I 

 possess specimens of P. dactylus which are worn quite smooth 

 towards the back of the valves by coming into contact with the 

 sides of the excavation, while the spines on the front remain 

 uninjured. This frequently happens with P. crispata and P. Can- 

 dida, which in the north of England are generally found in shale. 

 But the most conclusive evidence that the valves are not used as 

 rasping instruments is, that their anterior portions are frequently 

 covered with a tine epidermis. Montagu says that this is always 

 so in fine specimens of Teredo navalis. I have recently examined 

 more than one species of this genus, and find that the striated, — 

 that which is usually considered the cutting portion of the shell, 

 — is sometimes covered with a fine yellowish epidermis which can 

 be easily removed with the point of a penknife. The Pholades 



16* 



