438 BORING MOLLUSCA. 



and the whorls are sometimes so closely impressed on each 

 other, that the animal actually absorbs part of the tube 

 which it had previously deposited, in order to make room 

 for its new whorl. In one instance which has fallen under 

 my observation, it had left only a very thin transparent plate 

 between itself and the cavity of the tube. When, however, 

 the animal has nearly attained its full size the shell assumes 

 an irregular form, and is sometimes extended into a straight 

 line, and at others closely twisted over its former shell, 

 which, under such circumstances, it often absorbs. It is 

 not uncommon to find several young animals of this genus 

 burying themselves in the tube of an adult shell. 



These unequivocal instances of the power of the Mollusca 

 to dissolve their own shells, and to make holes in the shells 

 of other animals without the exertion of any mechanical 

 force, but by simply applying their mantle or foot to the 

 part to be dissolved, afford strong grounds for believing that 

 the holes formed by the regular boring Mollusca, such as 

 the Pholades, Petricolae, Venerupes, and Lithodomi, in shells 

 and calcareous rock, are produced in a similar manner ; and 

 this belief is strengthened by the following considerations : 



1st. That the animals of most of the boring shells are 

 furnished, like those of which I have just been speaking, 

 with a large foot, more or less expanded at the end. 



2ndly. That the holes bored by some of the Petricolse 

 and Grastrochaenae are compressed, and so exactly fit the 

 shell, that it would be impossible for the latter to rotate on 

 its axis in such a manner as to use the asperities of its sur- 

 face for the purpose of rasping, as some conchologists have 

 supposed. I have also seen specimens of Pholas pusillus, 

 the back valves of which were so much distorted, as to 

 demonstrate the impracticability of such a process, a pro- 

 jecting part of the back having evidently been fitted into a 

 cavity on one side of their cell ; yet these Pholades appeared 

 to have enlarged subsequently to the distortion having taken 

 place. Those of the Barnacles which bore, such as the 

 genera Conchotrya and Brismeus (and probably Lithotrya), 

 form an oblong compressed hole, of the exact size of their 

 shells. Specimens of Brismeus in my collection are placed 

 on the side of an oyster-shell, in which they have destroyed 

 part of two or three plates to form such a cavity ; but 

 although the shells of these boring Cirrhipedes are furnished 

 with raised lamellae, the projections are placed across the 

 valves in such a manner that no motion that could be given 

 to them would enable them to rasp a hole. 



3rdly. That all the boring shells are covered with a peri- 



