POWERS OF BURROWING. 347 



wearing agent. In the peduncle of an individual of 

 L. dorsalis, nearly ready to moult, I found, it may be re- 

 membered, beneath and round the basal disc, under the 

 old membrane of the peduncle, a new membrane studded 

 with calcified beads, but with the horny star-headed spines 

 not yet developed, whilst on the old outer coat these latter 

 had been worn down quite smooth, and the calcified beads 

 worn entirely away. Here, then, we have an excellent 

 rasping surface. With respect to the power of movement 

 necessary for the boring action, the peduncle is amply 

 furnished with transverse, oblique, and longitudinal striae- 

 less muscles, — the latter attached to the basal disc. In 

 all the pedunculata, I have reason to believe that these 

 muscles are in constant slight involuntary action. This 

 being the case, I conceive that the small, blunt, spur-like 

 portion of the peduncle, descending beneath the basal rim 

 of the lowest disc, would inevitably partake slightly of the 

 movements of the whole distended animal. As soon as 

 the Lithotrya has reached that depth, which its instinct 

 points out as most suitable to its habits, the discs are 

 converted into an irregularly growing cup, and the animal 

 then only increases in diameter, enlarging its cavity by 

 the action of the serrated scales on the peduncle, and of 

 the serrated lower edges of the valves of the capitulum. 

 With respect to those reversed individuals attached with 

 their capitulums downwards, I suppose that the larvae 

 had crept into some deep cavity, perhaps made originally 

 by a Lithotrya, of which the rock in the specimen in ques- 

 tion was quite full, and had there attached themselves. 

 Finally, it appears that in Lithotrya the burrowing is 

 simply a mechanical action ; it is effected by each layer 

 of shell in the basal attached discs over-lapping, in a 

 straight line, the last-formed layer, — by the membrane 

 of the peduncle and the valves of the capitulum having 

 excellent and often renewed rasping surfaces, — and, 

 lastly, by the end of the peduncle (that is homologically 

 the front of the head) thus roughened, extending beyond 



