As to our seaside pebbles, the most compact of them are even _visibly_ porous. Cut through the hardest jasper, and polish one face till it shines like a steel mirror, and then hold it so as to reflect the light, you will at once discern numerous minute specks or flaws on this surface, as if the point of a needle or graving-tool had been busily at work upon it. These specks have always been there. They are _air-holes_, and in the case of a fossil-pebble were, some of them, doubtless, connected with the position which the animal occupied just before he died. These air-holes must have traversed this hard stone in every direction, for, cut the pebble how you will, you meet with them. Others again, incomparably more numerous, but which can scarcely be discerned without the aid of a powerful microscope, are the true _pores_ belonging to this apparently impenetrable substance. I have found that the action of daylight tells even upon polished agates after a time. Some of the spots and markings in the finest stones grow fainter in the course of many years. Others appear to deepen. This is, no doubt, a change resulting from the oxygen, contained in a beam of light, acting readily upon carbon, silex, and other substances. And it furnishes an independent argument for concluding that the visible beauty of these formations, in perpetuating a coloured pattern, has been due rather to mixed _chemical_ processes than to pure crystallography. The fossil nodules, mostly siliceous, which we find upon a sea-beach, are _integral_ specimens. It never but once happened to me to meet with what appeared to be _two_ choanites in the same pebble, and I incline now to believe that this was in fact _one_ choanite who had _divided_ himself after the manner of the Luidia. But even supposing that here were, indeed, a pair of perfect animals, when it is considered that this was one single instance, occurring among more than a thousand picked up by me, in which the choanites were always solitary, such an exception may be almost said to prove the rule. The late lamented Hugh Miller has stated the like fact concerning his Morayshire and Cromarty icthyolites. At pp. 194, 195, of his “Cruise of the Betsy,” he remarks that “the limestone nodules take very generally the _form_ of the fish which they inclose; they are stone coffins carefully moulded to express the outline of the corpses lying within.” Then, after observing that the shape of the stone conforms to the attitude of the fish in every instance, he goes on to state that “the _size_ of the fish regulates that of the nodule. The coffin is generally as good a fit in size as in form; and the bulk of the nodule bears almost always a definite proportion to the amount of animal round which it had formed.” He then gives, in his usual clear and graphic language, the following remarkable statement, the result of direct experiment.