_Peculiarities in the Structure of the Radii._--In some of the species of Tetraclita, in which genus the walls consist of several rows of tubes, the radii are likewise perforated by several rows; and in some of the other species (Pl. 10, fig. 1 _h_), the edge, or disarticulated surface of the radius, is marked by irregularly branching ridges; and these evidently correspond with the branching septa or ridges of the wall. In Chelonobia, the outer lamina of the radius, as well as of its recipient furrow, is of extraordinary thickness; and this lamina, in _C. testudinaria_ (Pl. 14, fig. 1 _a_, 5, _b_, and Pl. 15, fig. 1, _f_), is modelled into sharp transverse ridges and valleys. In the Chthamalinæ, the radii, like the parietes, are simply solid; and apparently in consequence, for the sake of strengthening the sutures, the edges of the radii, and of the recipient furrow in Octomeris (Pl. 20, fig. 3 _a_) and in _Chthamalus dentatus_ and _Hembeli_ (Pl. 18, fig. 3 _b_, 5 _a_), are neatly dentated. In some other species of Chthamalus (Pl. 19, fig. 1 _a_), the radii present a slight modification of this structure, the sutures being formed by oblique interfolding laminæ. In the radii of Coronula and Tubicinella, there is a peculiarity, in apparent connection with the fact, that in these genera the parietal tubes are not crossed by transverse calcareous septa, namely, that the pores by which the radii are permeated keep unclosed throughout their length, and open into a special longitudinal tube (Pl. 16, fig. 7, _d′_), which runs along that margin of the wall, whence the radius arises. In Coronula the wall is of extreme thinness, and in conformity so is the true radius, but that the shell might not thus be rendered very weak, complementary or pseudo-radii are developed on their inner sides (Pl. 16, fig. 7, adjoining the true radii A _d_, C _d_, and shaded by distant convex lines). Even in the allied genus Xenobalanus, in which the whole shell tends to become rudimentary, traces of these pseudo-radii (Pl. 17, fig. 4 _b_, _d_) can be detected. In Coronula, though the radii (Pl. 16, fig. 7, A _d_, C _d_) are, by the above special means, rendered thick, and though the alæ also are thick (C _a′_, D _a′_), yet together they do not equal in thickness the folded walls; and consequently, there is left between the radii and alæ square chambers (_v_), occupied by the branching ovarian tubes. _Structure of the Alæ._