Fourthly, they were very different, as to their outward covering, some having the perfect Shell, both in figure, colour, and substance, sticking on upon its surface, and adhering to it, but might very easily be separated from it, and like other common _Cockle_ or _Scolop-shels_, which some of them most accurately resembled, were very dissoluble in common _Vinegar_, others of them, especially those _Serpentine_, or _Helical stones_ were cover’d or retained the shining or Pearl-colour’d substance of the inside of a Shel, which substance, on some parts of them, was exceeding thin, and might very easily be rubbed off; on other parts it was pretty thick, and retained a white coat, or flaky substance on the top, just like the outsides of such Shells; some of them had very large pieces of the Shell very plainly sticking on to them, which were easily to be broken or flaked off by degrees: they likewise, some of them retain’d all along the surface of them very pretty kind of _sutures_, such as are observ’d in the skulls of several kinds of living creatures, which _sutures_ were most curiously shap’d in the manner of leaves, and every one of them in the same Shell, exactly one like another, which I was able to discover plainly enough with my naked eye, but more perfectly and distinctly with my _Microscope_; all these sutures, by breaking some of these stones, I found to be the _termini_, or boundings of certain _diaphragms_, or partitions, which seem’d to divide the cavity of the Shell into a multitude of very proportionate and regular _cells_ or _caverns_, these _Diaphragms_, in many of them, I found very perfect and compleat, of a very distinct substance from that which fill’d the cavities, and exactly of the same kind with that which covered the outside, being for the most part whitish, or _mother of pearl_ colour’d. As for the cavities between those _Diaphragms_, I found some of them fill’d with Marle, and others with several kinds of stones, others, for the most part hollow, onely the whole cavity was usually covered over with a kind of _tartareous petrify’d_ substance, which stuck about the sides, and was there shot into very curious regular Figures, just as _Tartar_, or other dissolv’d Salts are observ’d to stick and _crystallize_ about the sides of the containing Vessels; or like those little _Diamants_ which I before observed to have covered the vaulted cavity of a Flint; others had these cavities all lin’d with a kind of _metalline_ or _marchasite-like_ substance, which with a _Microscope_ I could as plainly see most curiously and regularly figured, as I had done those in a Flint.