bivalve shell (for the Pholas, with the exception of the accessory valves, is in all other respects a bivalve)? Fig. 26. will explain. The part marked (a) is a tough elastic horny substance,

which connects the two valves on one side of the summits or bosses (umbones) of the shell (b), and is named the Ligament or Cartilage. This is so constituted, that, by its elasticity, the valves have a constant tendency to open. They are kept shut by the contraction of a muscle inside; and hence, when, in opening an oyster or a scallop, we cut through that muscle the shells

separate to a considerable distance. When the animal also is so long out of its native element, that it is thereby greatly weakened, and the contractile power of the muscle diminished, we observe that the animal lies with the shells open, the elasticity of the ligament being too powerful for the debilitated contractility of the muscle. Now the part where the cartilage or ligament is placed is the anterior; while, of course, the other side is the posterior, end.

You may next, perhaps, think it rather singular that the shell should be described as so much broader than it is long; but you must learn, that the length of a shell means the distance from the hinge to the base, and, consequently, the length may either be the longer or the shorter diameter, according to the species. The muscle, for example, (fig. 26. c.) is as long again as it is broad; while in the pod razor shell (Solen Legumen) (fig. 26. d.) the breadth is nearly four inches, and the length scarcely three quarters of an inch. The length of a shell, then, does not mean its longest diameter, but the space from the hinge to the base; and, consequently, it varies exceedingly; in some shells being longer, in some much shorter, and

in others neither longer nor shorter than the breadth, as in some species of Arca. (fig. 26. e.)

There are six British species of Pholas, and about a dozen or fourteen known altogether; Dillwyn describes eleven. You will recollect that in them all, the accessory valves are very deciduous, that is, they readily fall off; they are also easily broken; and hence, in specimens not well preserved, they are often wanting, but still, by the single incurved tooth in each valve, together with the general aspect, you will readily recognise them, so far at least as the genus is concerned. is concerned. Some species of Pholas are used as food, and also by fishermen as a bait. But the most remarkable fact in the history of the Pholades, is the power which they have of lodging in rocks and wood, even the hardest oak. This property, however, is not confined to them, for there are other species of testacea which form similar lodgments, in a manner equally inscrutable; such are several of the Mya genus, the Donax Irus, or foliated wedge-shell, some Venus shells, the burrowing, cross-beaked, coral-piercing, and other muscles, besides some more bivalves; but I am not aware that any of the univalves have a similar property. These animals have no mechanical instruments for boring, and much conjecture

has been used relative to the mode by which they can accomplish so apparently arduous and difficult a task as forming their cells. Most, if not all of them, secrete a luminous fluid; and I believe the most general opinion is, that this luminosity is connected with or caused by phosphorus. Dr. Turton observes, that the three species of pholas (dactylus, our present one; parva, or small piddock; and candida, or white piddock,) are found in vast numbers in masses of rock, taken at the mouth of the river just below the town of Teignmouth in Devonshire; and he remarks, that "the philosophy of their natural history may probably be of no very difficult solution. The rock in which they are imbedded is a cementation of the finest sand and lime, and of so very soft a substance when first taken from its bed, as to be easily cut with a knife into any form, and sufficiently absorbent to afford moisture for the purposes of life and their peculiar action. The animals themselves abundantly secrete a mild phosphoric solution, as may be seen by its illuminating in the dark whatever is moistened with it, sufficiently powerful to decompose the rock by the slow contact of their gradually increasing bulk. The atmospheric air also seems to be occasionally necessary to this process, as they are always found in situations which are left dry for

a short period by the recess of the lowest tides, its oxygen perhaps serving by its union with this secretion to form a phosphorous acid. In confirmation of this belief, we have affixed them, when fresh taken, to a smooth piece of the same rock, by the frontal gape, occasionally moistening them with sea-water; and in a few days have found, that at the place of contact an evident waste of substance had been made by the decomposition of the lime and a deposition of sand in the finest grains. It may reasonably be supposed that all the borers of rock and wood, even the teredo, act in this manner by their peculiar and appropriate solvents." *

I am afraid that this throws very little light on the subject. We have no proof that the luminous secretion of these animals is owing to phosphorus; and we know that many species which have the luminous, are quite destitute of the burrowing property; and, besides, the phosphoric acid would be about the worst agent which could be selected, because the phosphate of lime is insoluble in water, and I should fear, that when the carbonate was decomposed, the phosphate produced, instead of being carried off, would be deposited, and form a more intractable substance than the original chalk.