GENUS CORONULA. 403 



C. balanaris, however, the inner hood-like ends of the 

 folded walls are not produced so much downwards as is 

 usual. 



In the same manner, as the outside of the shell consists 

 of the transversely expanded ends of the folded walls, 

 pressed closely together, so the cavity in which the animal's 

 body is lodged, is formed by the inner and less closely joined 

 ends of the folds, lined by the thick sheath (a, fig. 1 and 7, 

 PI; 16), which latter extends down very near to the basal 

 membrane. The cavity for the body, is small compared to 

 the whole shell; in C. diadema it is deeply cup-formed, 

 with a small, flat, membranous bottom or basis ; in C. balce- 

 naris it is wider and shallower, with a broader bottom, and 

 with the upper edges of the walls more inflected. In both 

 species, the thick membrane connecting the opercular valves 

 to the shell, is attached all round near the summit of the 

 sheath. The uppermost portion of the sheath is not marked 

 by concentric lines, as in most of the Balanidae, owing to 

 the opercular membrane not being, as we shall presently 

 see, regularly moulted. A portion of a single wall, when 

 closely examined, is found to be formed of an outer and 

 inner lamina, united by longitudinal septa, and is thus per- 

 meated by minute, square, longitudinal pores, — exactly as 

 in the normal structure of Balanus. The walls are extremely 

 thin, and are striated longitudinally, owing to the slight pro- 

 jection, on both the inner and outer surfaces, of the longi- 

 tudinal septa; they are thicker in the part forming the 

 external transverse loops, being here, in C. balanaris, as much 

 as Cloths of an inch in thickness; but when forming one 

 side of the spoke-like folds, the thickness is only ^ths of an 

 inch. The inner lamina is thicker, contrary to what is 

 usual, than the outer lamina; the sharp tips of the longitu- 

 dinal septa project a little beyond either lamina, giving to 

 the basal edge of the wall a serrated outline. It is singular 

 that the thin outer lamina is first formed as a rim or ledge 

 on each side of the longitudinal septa ; these ledges being 

 not closely united for some little space up the wall, as is 

 represented in the enlarged drawing of a bit of the basal 

 wall of C. diadema, PI. 16, fig. 6. The open clefts thus 

 left are, of course, covered by the so-called epidermis, for 



