• TO A. HAXCOCK, ESQ. 261 



No. 8. 

 (Postal date Dec. 28tli, 1850.) 



Down Farnborough, Kent, 



Dec. 25th. 

 My dear Sir, 



As you Iiave attended with, such eminent success to the 



boring of animals into rocks, you will perhaps like to hear that 



I believe I now understand the boring of Lithotrya, thanks to the 



enclosed drawing (which please return) sent me by Steenstrup 



without text. I suppose the same explanation is applicable to 



Arthrobalanus (=z CryptopMalus, Darwin), and I should think 



Alcippe (for the presence of the calcareous disc is not material 



to the change of place), but not, as far as I can see, to Clitia. 



Since receiving this same Plate I have had a good deal of 

 rock, bored by L. dorsalis, given me, and I now find out, for the 

 first time the following important facts: — (1) That the animal 

 bores to its full depth when young, and afterwards only increases 

 the diameter of its hole. 2nd, That a cup is only formed when 

 the animal has ceased boring to a greater depth; but that before a 

 cup is formed, a succession of little discs, exactly as represented, 

 are deposited on one side of the hole, each new one, at each 

 fresh exuviation, being placed -aVth or -A-th of an inch, or even 

 more, beneath that last formed ; the disc or cup, as I was always 

 certain, never itself being moved. 3rd, The lowest disc is never 

 at the bottom of the burrow, and this is faithfully represented 

 in the Plate. Lastly, the skin of the peduncle at this lottommost 

 part, at first, after each exuviation, is studded with minute cal- 

 careous beads, which are soon fairly worn away ; and the beads 

 are succeeded by hard horny star-headed points, which are also 

 much worn away before a new moult. So that there is good 

 wearing agency. (I^.B. — I found specimens with perfect coat 

 underneath old coat nearly ready to moult, so no possible mis- 

 take.) I should have said that as soon as the animal begins to 

 increase much in diameter the chain of little discs are of course 

 all worn away, so that no trace is left in full-sized specimens. 



In the drawing you will at once understand how the animal 



