clusion that the greater thickness of the earlier and older growths of the shell is owing to the circumstance that, on the addition of each new segment, the cell-wall by which it is enclosed has not terminated at the boundary of the individual segment itself, but has been prolonged over a considerable number, if not the whole, of the segments belonging to the outermost convolution of the animal. But before the newly-formed segment enclosed itself within the limits of its own calcareous shell, it must have had the power of spreading itself out in the form of a thin gelatinous layer, investing the whole of the preexisting organism." 1 That is to say, whenever the animal has so increased in size, in the last-formed chamber, that a new chamber must be constructed, the sarcode diffuses itself over the whole external surface of the shell, and deposits a new layer of calcareous matter over all the shell, then retreats within the last segment again, and completes a new segment, which becomes the last, and is the thinnest, because it has had no external stratum deposited upon it. Moreover, it is known that, at certain times, the sarcode does envelope the entire shell, and this explains wherefore it is done.

This hypothesis explains, and accounts for, the

increasing thickness of the walls of the various chambers, from the latest back to the earliest, the increased thickness being due to the successive deposition of thin external strata of calcareous matter, which strata would consequently be the most numerous in the oldest chambers, diminishing regularly and consecutively towards the most recentlyconstructed chamber. "Having so spread itself out as an investing body, it appears to have deposited new calcareous layers, covering over the greater part of the pre-existing external segments; but in each layer so added open points have been left opposite to the mouths of the pseudopodian tubes in the layers previously formed, reminding us of the way in which similar apertures are left opposite the mouths of the canaliculi in membraniform bone-growths. In some instances, however, especially near the umbilical region, these tubes have been blocked up by the more recent investments."

We might also refer to the shell structure itself, and especially in those species in which "the outer layers are perforated in every direction by a network of minute anastomosing canals, which communicate freely with the exterior of the shell through numerous minute apertures." All these minute details of elaboration are marvellous, especially in the construction of the skeleton of animals so small, and of such simple organisation, that they are placed very near the bottom in the graduated scale of animal life,

"How each fulfill'd

The utmost purpose of its span of being,
And did its duty in its narrow circle,
As surely as the sun, in his career,
Accomplishes the glorious end of his."

It would be almost hopeless to attempt any verbal description of all the various forms of shells which are constructed by the different species of Foraminifera.. Within certain limits, and in this instance very broad ones, each species has its own type of shell. This was more true in the days when the form of the shell was the principal character than it is at present, but there are some broad general features which find acceptance now. Primarily, there are three distinct varieties of texture the porcellanous, resembling porcelain; the hyaline or vitreous, which are more glassy; and the arenaceous, or sandy. In the first the texture bears a strong resemblance to porcelain, especially when the surface is highly polished. Some of these are ribbed, others are channelled, whilst others are pitted either with small depressions or large areolæ. The vitreous shells have almost a glassy transparency, sometimes opaline, and these, again, may be sculptured on the surface with ridges or tubercles. Dead shells, when subject to long action of sea-water, lose their lustre and become opaque. Finally, the arenaceous are shells composed, partly at least, of particles of sand, obtained externally and agglutinated together by a cement supplied by the animal. The fine particles thus collected vary considerably, both in colour and substance. Sometimes the little particles are very uniform and methodically disposed; at others, they are less uniform, and form only an outer layer imbedded in a calcareous base. It seems a mystery how such a simple animal as that which inhabits these shells, little more than an atom of living jelly, can deposit one chamber upon another, in the multilocular shells, with the characteristic markings sculptured on the surface, or select and collect the minute particles of sand, and bind them together with admirable uniformity, like a miniature tessulated pavement, on the exterior of the arenaceous shells. Truly "there are more things in heaven and earth,"and the deep sea also, -" than are dreamt of in your philosophy."

The simplest form of Foraminiferous shell is that in which the house is restricted to a single chamber. This may be as nearly as possible of a globose form, or oval, or attenuated at one end, so as to be almost pear-shaped, or the neck may be elongated so that it resembles a Florence oil-flask, some being plain, others ribbed or chequered. More complex forms consist of a series of spheres or ovals, adhering in a row, of which each successive chamber diminishes towards one end. In other forms the chambers approximate so closely that they are flattened at the poles, and united without any intervening isthmus. These may be straight or curved, but still gradually attenuated to one end. From the straight or curved forms the transition is easy to those in which the smallest end is curled into the commencement of a spiral. Then we encounter more

perfect spirals, as complete as in the shell of a snail, but always with more or less distinct transverse lines, or furrows, indicating the division into chambers. Sometimes these spiral shells are nearly smooth, or they may be wrinkled, ribbed, indented, perforated, sculptured in

FIG. 5.-AGATHISTEGA.

*

FIG. 6.-HELIXOSTEGA.

various ways, spurred, or keeled. In some the terminal opening is large, in others small. Then to all