Fig. 18 illustrates the relative disposition of these branches. In _a_, the natural size of the fossil is given; _b_ is a portion of the same magnified, to show the form and arrangement of the ribs and cross-bars. Each rib is seen to have two sides separated by a rounded ridge. Along each side there runs a row of circular hollows or cells, every one of which once formed the abode of a distinct bryozoon. The back or inner surface of the branch, was ribbed and granulated irregularly, without any cells. The connecting bars or dissepiments have no cells, and served merely to bind the interstices together into one firm organically-united polypidom. Such fragments as that here figured are the most usual traces to be found of these animals among the carboniferous rocks. But perfect specimens are sometimes met with which show how delicate and graceful a structure the polypidom of some of the fenestellæ must have been. All these bars sprung from a common point as their basis, and rose up in the form of a cup. It was, in short, a cup of network, hung with waving tentacles and quivering cilia. I have seen some dissections of flowers in which all the softer tissue had been removed, so as to present only the harder veinings of the leaves with their thousand ramifications bleached to a delicate whiteness. Out of these skeleton-leaves there were formed groups of lilies, crocuses, geraniums, and roses, like patterns of the finest gauze. Some of the larger-stemmed leaves that had been artistically moulded into a tulip form, seemed not inaptly to represent the general contour of the skeleton of the old carboniferous fenestella. An allied form is called the _Retepora_. It differed from the previous organism in having the ribs not straight, but irregularly anastomosing, that is, running into and coalescing with each other, so as to form a close network with oval interspaces, like a piece of very minute wire-fence. Each of these wavy libs was completely covered over on one side with oval pores or cells, which, as in the fenestella, formed the abode of the living animals. The differences in organization between the animal of fenestella and that of retepora can, of course, only be matter of speculation. The general structure in both must, however, have been pretty much alike. The former genus is now no longer extant, but the latter, which was ushered into the world during the era of the Old Red Sandstone, still lives in the deeper recesses of the ocean, and manifests in its structure and habits the leading characteristics of bryozoan life.