The organic remains hitherto described belong to that large division of the animal kingdom instituted by Lamarck, to comprehend all those whose internal structure is supported by no vertebral column, and which are hence termed invertebrate. They are for the most part protected by a hard outer covering, or exo-skeleton, which assumes many different modifications. We have seen it in the calcareous cells of the little net-like fenestella, in the geometric cup of the stone-lily, in the double case of the cypris, and in the shells of the mollusca. But the order of nature does not always exactly correspond with the classification of the naturalist. His system must necessarily be precise, formal, and defined. One tribe ends off abruptly, and is immediately succeeded by another, with different functions and structure, and dignified with a separate name. But in the order of creation, such abrupt demarcations are few, for if they exist in the present economy, they can not unfrequently be filled up from the existences of the past. There is usually a shading off of one class into another, like the blending of the tints of sunset, and it often baffles all the skill of the profoundest anatomist, by drawing a distinct line, to pronounce where the one division actually ends and the other begins. Any name, therefore, which is intended to embrace a large section of the animal kingdom, must ever be more or less arbitrary. It will extend too far in one direction, and embrace organisms which might be classed in a different section. It will probably not extend far enough in another, and thus leave beyond its pale animals possessing strong affinities to the majority of those included under it. More especially is this true of every system of classification that proceeds upon the modifications of a single feature, or upon mere negative resemblances. Suppose, for instance, that it were proposed by some highly systematic individual to divide the inhabitants of our country into two great classes--the bearded and the beardless. In the latter category he would arrange all the more quiet and orderly portion of the community, with perchance a tolerable intermixture of rogues. The bearded group would present a most motley array--from the fierce-visaged heroes of the Crimea to the peaceable stone-mason or begrimed pitman--all brought into one list, and yet agreeing in no single feature save that of being like Bully Bottom the weaver, "marvellous hairy about the face." But Lamarck's invertebrate division of the animal kingdom presents a grouping of yet more diverse characteristics, as cannot fail to be confessed when we recollect that it embraces among its members the microscopic monad, the coral polyp, the lobster, the butterfly, the limpet, the nautilus, and the cuttle-fish. Cuvier's three-fold grouping of the division into _mollusca_, _articulata_, and _radiata_, has now supplanted the old name, though the latter is still retained as a sort of convenient