Although the character and structure assumed by the semi-fluid bodies of the known Foraminifera have been determined in most cases with admirable precision, it is still thought advisable to arrange them according to the substance of the shell: consequently they form three natural orders; namely, the Porcellanous or imperforate, which have calcareous shells often so polished and shining that they resemble porcelain; secondly, the Arenaceous Foraminifera, consisting of animals which secrete a kind of cement from their surfaces, and cover themselves with calcareous or siliceous sand-grains; and lastly, the Vitreous and Perforated order, which is the most numerous and highly organized of the whole class, has siliceous shells transparent as glass, but acquires more or less of an opaque aspect in consequence of minute straight tubes which perforate the substance of the shell perpendicularly to its surface, and consequently interfere with the transmission of light. _Order of Porcellanous Foraminifera._ The Miliolidæ constitute the porcellanous order, which consists of twelve genera and many species, varying from a mere scale to such as have chambered shells of complicated structure. The genus Miliola has minute white shells resembling millet seeds, often so brilliantly polished that they are perfectly characteristic of the porcelain family to which they belong. No Foraminifera are better suited to give an idea of the intimate connection between the shell and its inhabitant than the Miliola, the fundamental type of this genus. The shell is a spiral (I, fig. 97), which is made up of a series of half turns arranged symmetrically on its two sides. Each half turn is longer and of greater area than that on the opposite side, so that each turn of the spire has a tendency to extend itself in some degree over the preceding one, which gives a concave instead of a convex border to the inner wall of the chamber. The sarcode body of the Miliola consists of long segments which fill the chambers, connected by threads of sarcode passing through the tubular constrictions of the shell. As the animal grows, its pseudopodia extend alternately now from one end, and now from the other extremity of the spiral, and by them it fixes itself to seaweeds, zoophytes, and other bodies, for these Foraminifera never float or swim freely in the water. The genus Miliola is more extensively diffused than almost any other group of Foraminifera; they are most abundant between the shore and a depth of 150 fathoms, and are occasionally brought up from great depths. Beds of miliolite limestone show to what an extent the Miliola abounded in the seas of the Eocene period; but the type is traced back to the Lias.