176 PHILOSOPHICAL TBANSACTION8. [aNNO J 7 65. 



are 1 depressions, which terminate in 1 holes in the bases of the great valves, 

 which are half covered by the posterior circular piece. This apex is round and 

 flattish, and forms almost a sharp edge by the concurrence of the 1 great valves, 

 and the fore and back edges are united by the long pieces before-mentioned. 



The texture of the shell is very thin and brittle; so that it is wonderful to see 

 the holes they lie in so smooth and uniform, as if bored with a hard sharp instru- 

 ment. The base end is always inward, and the hole, which opens from them 

 outwards, very small; and this is the case of every kind which are thus lodged, 

 whether in wood or stone ; so that we must conclude that they are deposited there 

 in a very minute state, and not in a state of maturity; for then they must bore 

 their way inwards, and the hole would be as wide outwards as inwards, and con- 

 sequently be of equal diameter. But how these animals maintain and increase 

 the cavity, as they grow larger, is a question which it will be very difficult to 

 resolve, and has puzzled several ingenious naturalists in the inquiry. 



It is said they have a power of turning themselves about with a swift motion, 

 and so gradually make themselves room ; but this will be hard to conceive, if 

 we consider that a fish closely shut up within its valves, and compressed on all 

 sides, can have no power of motion. We cannot imagine any animal can move 

 itself, when thus confined, without some fulcrum or point of efi^brt, from which 

 to begin such motion ; and if they had such tentacula as were capable of seizing 

 on the wood, in order to exert themselves, there can be no room for it, for it is 

 in close contact with themselves in every point. That this is the case is very 

 clear, from considering the state of toads, frogs, and other animals, inclosed in 

 blocks of marble, trunks of trees, &c. which have no communication with the 

 atmosphere at all. These are soft animals, and their shape not at all fit for turn- 

 ing about and boring their cavities: and yet they are found in moulds as exactly 

 fitted to their bodies, as those are to melted matter cast into them by a founder. 

 It may however be supposed that the stone and wood does actually give way to 

 the growth of the animal within, because the facts are well attested; but how 

 this comes to pass, in these pholades and cylindrical muscles, and by what means 

 toads, &c. can receive aliment to cause their growth, without any external com- 

 munication, must yet remain among those secrets of nature, which we cannot 

 but admire, without knowing how they are brought about. 



Fig. 3, pi. 5, is a view of the surface of one of the great valves, with the 

 edges of the two longitudinal pieces, and with portions of the smooth parts at 

 the round extremity or base. Fig. 4, shows the anterior edge of the pholas 

 covered by the long smooth white piece, and at the base having part of the 

 smooth portions of the great valves in view. Fig. 5 represents the posterior edge 

 of the pholas, with the round white piece on the base end, and the long ditto, 

 which is larger than that of the fore edge, running towards the apex. Fig. 6 is 



