Sometimes, on our beaches, hollow globes of flint are picked up, which, when you break them open, are found to be full of a white, powdery substance, like the chemist’s magnesia. This is much the same as the “rock-milk” of mineralogists, a very fine deposition of lime, occasionally met with in beds of the chalk strata, and considered to be a result of some filtering process, when the water, after being long pent up, had escaped by small crevices. But in the flint-globes no such straining can have taken place; and in sundry specimens which I have examined, I generally came to the conclusion that some conchiferous animal had been inclosed in the nodule, and his shell had afterwards broken and been pulverized. Sometimes I found within the “rock-milk” a dark-brown, carbonaceous spot; this would be the creature’s body, as Dr. Mantell supposes of his “molluskite.” The cavity is never quite filled up. I imagine many of these “powder-horns” to be altogether modern in their date. A much more curious phenomenon than the above occurs from time to time in solid agate-pebbles, which, when picked up, are found to contain in a central chamber, visible to the eye, a drop of water. These are scarce, and fetch a high price at the museums. As much as twenty pounds has been more than once given for an undoubted specimen. How the above-mentioned singular effect was arrived at, has, I believe, never been explained; at least, not so as to satisfy a close reasoner. The same _lusus naturæ_ is met with, I think more frequently, in rock-crystal and in the dark fluor-spars. The last apparent animal organism which I shall notice, as having more than once occurred to me as a _possible_ explanation of the patterns disclosed in certain sections of agate-pebbles, is that of some creature’s “ovary.” The eggs of the whelk, and those of the cuttle-fish, are deposited in clusters, like some of the “grapes” on sea-weed. I have found, in some of the Isle of Wight pebbles, a conformation closely resembling this, but on a much smaller scale. That the original substance was part of an animal, I have no doubt whatever; and the arrangement of the lobes or spherules, composing the mass, more resembles that of the spawn of the above-named fishes than anything else which I am acquainted with. Still, resemblance is not identity. The thing may be only a curious coincidence, resulting from some unexplained freak of nature; as when she carved the profile of Napoleon’s face in the outline of a part of Mont Blanc, and also (quite as distinctly) in that of a hill which overlooks the town of Belfast. [5] See his Letter to Dr. Burnet. REFERENCES TO THE CHROMO-PLATES. In the frontispiece is represented the polished section of a pebble which the author picked up on the beach at Bonchurch, in the Isle of Wight. This is an unusually large and perfect specimen, the body of the Choanite lying nearly central. The pebble contains one or two blotches of _native iron_. The “cuticle” is uninjured.