136 BALANTD.E. 



represented in PL 28) extending under the doubled walls 

 and terminal transverse loops. It has eighteen concave 

 sides, corresponding with the inner ends of the folded walls, 

 for each of the six compartments is trebly folded. The 

 membrane consists of successive, conformable slips [c\ c), 

 bordered exteriorly by thickened yellowish rims, and 

 internally overlapping (when viewed from the inner side) 

 the few last-formed slips, and then thinning out. The 

 membrane forming each slip is itself laminated. The 

 middle portion, about ^th of an inch in diameter, is rather 

 opaque, owing to the slips being so close together. Beyond 

 this central part, the slips suddenly increase in size, but yet 

 have a different shape from the 18-sided outline, which 

 they ultimately assume : this difference is owing to the 

 great changes in shape, as explained under the genus 

 Coronula, which the shell undergoes, when the Avails at tirst 

 assume their folded structure. The walls are invested by 

 longitudinally striated membrane {p,p,p, fig. 1 a), which 

 turns in under their basal edges; and this membrane is 

 united with the basal membrane, by what I shall call the 

 circumferential slip (b), and which is shaded in fig. 1 a, 

 simply for the sake of catching the eye. It is the circum- 

 ferential slip of membrane which sends rays under the 

 spoke-like folded walls : thin as it is, this slip is yet lami- 

 nated, but is not bordered by thickened edges. The mem- 

 brane investing the walls is, like the basal membrane, 

 formed of successive slips with thickened edges, which over- 

 lapping (viewed from the inside) the last-formed slips, project 

 beyond them, and so face the edges of the slips in the basal 

 membrane ; they are only obscurely indicated in fig. 1 a. 

 The circumferential slip {b) lies over (as viewed from within) 

 both the basal and wall membrane. This whole structure 

 will, perhaps, be best understood by the sectional diagram 

 (fig. 1 b), in which the letters (c, c) show the slips of basal 

 membrane ; (p) the parietal membrane, coating the outside 

 surface of the walls of the shell, not here represented ; 

 {b) the circumferential slip overlying both ; and [z, z) the 

 layers of cement, which may for the present be disregarded. 

 In order to allow of the growth of the shell, the circum- 

 ferential slip (b) periodically splits in the middle, all round, 



