STRUCTURE OF SHELL. 47 



the wall. In Chelonobia, the outer lamina of the radius, 

 as well as of its recipient furrow, is of extraordinary thick- 

 ness; and this lamina, in C. testudinaria (PL 14, fig. 1 a, 5, b, 

 and PL 15, fig. 1,/), is modelled into sharp transverse 

 ridges and valleys. In the Chthamalinae, 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 laminae. In the radii of Coronal a 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 Ad, 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, ac/, cd) are, by the above special means, rendered 

 thick, and though the alse also are thick {ca,j)a , ) 9 yet 

 together they do not equal in thickness the folded walls; and 

 consequently, there is left between" the radii and ake square 

 chambers (v), occupied by the branching ovarian tubes. 



Alee. — These project, generally abruptly, from the sides 

 of the upper part of the compartments ; they appear from 

 the first growth of the shell ; they are overlapped by the 

 radius and by part of the wall of the adjoining compart- 

 ment; they are thinner, and have, owing apparently to 

 being overlapped, a very different aspect from the parietal 

 portion ; but they do not differ from it in essential nature. 



