we arrive at the important conclusion that the strata in which the remains of cyprides abound must have been deposited in lakes or rivers. This gives us a key by which to interpret some of the changes of a geological system, and the ancient physical revolutions of large tracts of country. The shales of the coal-measures sometimes contain the cypris cases in such abundance as to derive therefrom a sort of fissile structure. It should be borne in mind, however, that each animal may during its lifetime have possessed in succession several of these cases. Among the shell-bearing molluscous animals, the little shell which contains the creature in its youngest stages remains ever after as an integral part of the outer calcareous case. As the inhabitant grows, it continues to add band after band to the outer edge of the shell, each of which, whilst preserving the general symmetry and proportions of the whole structure, increases its dimensions in every way. Among the univalves, such, for example, as the turritella, so common on our shores, the layers of growth succeed each other like the steps of one of those long spiral stairs that our feudal forefathers loved to build from the court-yard to the watch-tower of their castles. Each new layer exceeding in bulk its predecessors, adds a new step to the ascending pile, and thus the ever-widening mouth winds spirally upwards around the central pillar. The bivalves exemplify the same principle. The successive additions are made in a crescent form to the outer edges, and form those prominent concentric ridges so conspicuous on many of our commoner shells, such, for instance, as some of those in the genera _Astarte_ and _Venus_. But the architecture of the Crustacea (and, of course, that of the cyprides) is conducted on a very different principle. Their houses admit of no additions or enlargements, and so, when the animals find themselves getting somewhat straitened, they retire to a sheltered spot, and there, separating the walls that hem them in, crawl out like soft lumps of dough. The outer membrane of the moulted animal quickly acquires strength and hardness, and in a day or two the renovated creature is as healthy and vigorous as ever. In this process it is not merely an external shell, like that of a mollusc, which is thrown off, but a veritable skin, so that when the old shell is abandoned it frequently could not be detected on a first glance to be empty, the outer crust of every leg and joint, and sometimes even of thin bristles, remaining just as in the living animal.