552 LEPADID.E. 



yond and above the homy disc, only just enough to wear 

 away the surrounding shell to the thickness necessary to 

 allow of the formation of each new zone of disc; as the disc 

 itself is not armed, it subsequently has no power of wearing 

 away the plate of shell above it. Thus the horny disc, 

 besides giving support and attachment to the peduncle, is 

 of this peculiar service that it seems to guide, (somewhat 

 like the wood-part in a plane), the rasping powers of the 

 lower extreme margin of the peduncle. 



I may here observe that certain radiating and often 

 punctured lines, mentioned and figured by Mr. Hancock, 

 which help to render the thin plate of shell over the peduncle 

 conspicuous (tig. 3), are formed by the burrows of an ex- 

 cessively minute annelid, the punctures being apparently 

 the exit orifices: I imagine that these annelids find it difficult 

 to commence their burrows on the smooth surface of the shell, 

 and that they congregate at these particular spots and 

 thence burrow in radiating lines, owing to their bavins 

 taken advantage of the little cliff-like edges, at the narrow 

 and disused ends of the fissures leading into the cavities 

 occupied by the Alcippe, where alone they would not be 

 disturbed by the action of the cirri, when first they com- 

 menced making their little burrows in the shell. 



The fissure leading into the cavity is required to be broad 

 at the posterior end, in order that the cirri may be there freely 

 exserted out of the sack; and narrow in other parts, to 

 prevent, as it would appear, anything injurious getting in 

 between the animal's body and the cavity in the shell of the 

 mollusc. As the fissure is increased in length by attrition 

 at the broad posterior end, which end during growth becomes 

 broader and broader, the lower part of the fissure has to be 

 narrowed, and this is effected in a very singular manner, 

 namely, by advantage being taken of the strong tendency, 

 which triturated shell with animal matter, has to set into 

 a solid shelly mass, although constanty agitated.* Mr. 



* I have given some remarkable cases in my volume on 'Volcanic Islands,' 

 (p. ¥.)), in which limestone, having almost the hardness and specific gravity of 

 marble, has been thus deposited. Almost every coral-reef offers similar examples. 

 The curious substance described by Mr. Horner and Sir David lircwstcr, 

 [* Philosoph. Transact./ L836, p. 65), which is formed during the manufactory 



