In the course of his observations in Italy, Stensen had seen many mussel shells, which had been gathered from various layers of the earth's surface. With regard to the shells themselves, he said that there could be no doubt that they had come as the excretion of the mantle of the mussel, and that the differences that could be noted in them were in accordance with the varying forms of these animals. He pointed out, however, that some of the mussel shells found in {158} strata of rock were really mussel shells in every respect as regards the material of which they were composed as well as their interior structure and their external form, so that there could be no possible question of their origin. On the other hand, a certain number of the so-called mussel shells were not composed of the ordinary materials of which such shells are usually made up; but had indeed only the external form of genuine shells. Stensen considered, however, that even these must be regarded as originating in real mussel shells, the original substance having been later on replaced by other material. He explained this replacement process in very much the same way that we now suggest the explanation of various processes of petrification. There is no doubt that in this he went far beyond his contemporaries, and pointed out very clearly what was to be the teaching of generations long after his own. The same principles he applied to mussel shells, Stensen considered must have their application also to all other portions of animal bodies, teeth, bones, whole skeletons, and even more perishable animal materials that might be found buried in the earth's strata. His treatment of the question of the remains of plants was quite as satisfactory as that of the animals. He distinguished between the impressions of plants, the petrification of plants, the carbonization of plants, and then dwelt somewhat on the tendency of certain minerals to form dendrites, that is, branching {159} processes which look not unlike plants. He pointed out how easy it is to be deceived by these appearances, and stated very clearly the distinction between real plants and such simulated ones.