The shells require special study. In the lowest forms they are membranous, sometimes encrusted with sand-grains, always very simple, the only complication being the doubling of the pylome in _Diplophrys_ (fig. 2, 1), _Shepheardella_ (fig. 2, 3-5), _Amphitrema_ (fig. 2, 11), _Diaphorophodon_ (fig. 2, 12). The marine shells are, as we have seen, of cemented particles, or calcareous, glassy, and regularly perforated, or again calcareous, but porcellanous and rarely perforate. These characters have been used as a guide to classification; but some sandy forms have so large a proportion of calcareous cement that they might well be called encrusted calcareous genera, and are also not very constant in respect of the character of perforation. The porcellanous genera, however, form a compact group, the replacement of the shell by silica in forms dwelling in the red clay of the ocean abysses, where calcium carbonate is soluble, not really making any difficulty. Moreover, the shells of this group show a deflected process or neck of the embryonic chamber ("camptopyle") at least in the megalospheric forms, whereas when such a neck exists in other groups it is straight. The opening of the shell is called the pylome. This may be a mere hole where the lateral walls of the body end, or there may be a diaphragmatic ingrowth so as to narrow the entrance. It may be a simple rounded opening, oblong or tri-multi-radiate, or branching (fig. 4, 1); or replaced by a number of coarse pores ("ethmopyle") (fig. 3, 5a). Again, it may lie at the end of a narrowed tube ("stylopyle"), which in _Lagena_ (fig. 3, 9) may project outwards ("ectoselenial"), or inwards ("entoselenial"). In most groups the stylopyle is straight; but in the majority of the porcellanous shells it is bent down on the side of the shell, and constitutes the "flexopyle" of A. Kemna, which being a hybrid term should be replaced by "camptopyle." The animal usually forms a simple shell only after it has attained a certain size, and this "embryonic chamber" cannot grow further. In _Spirillina_ and _Ammodiscus_ there is no pylomic end-wall, and the shell continues to grow as a spiral tube; in _Cornuspira_ (fig. 3, 1) there is a slight constriction indicating the junction of a small embryonic chamber with a camptopyle, but the rest of the shell is a simple flat spiral of several turns. In the majority at least one chamber follows the first, with its own pylome at the distal end. This second chamber may rest on the first, so that the part on which it rests serves as a party-wall bounding the front of the newer chamber as well as the back of the older; and this state prevails for all added chambers in such cases. In the highest vitreous shells, however, each chamber has its complete "proper wall"; while a "supplementary skeleton," a deposit of shelly matter, binds the chambers together into a compact whole. In all cases the protoplasm from the pylome may deposit additional matter on the outside of the shell, so