STRUCTURE OF SHELL. 45 



(see transverse section, PL ] 6, figs. 5,7) is very remarkable 

 from being deeply folded, the folds being on their internal 

 faces firmly calcified together, and on their external faces 

 closely pressed together (often with a neatly serrated 

 suture), so that the whole nature of the shell might be, as 

 has happened, easily quite misunderstood ; and the walls be 

 considered as very thick, instead of being, as is really the 

 case, very thin. In Chelonobia (PI. 15, fig. 1), however, 

 the walls are truly of such great thickness, that the nature 

 of the relative parts might likewise be misunderstood ; in 

 this genus the ovarian tubes enter the walls, extending up 

 between the longitudinal septa, or, as they may here 

 be more naturally called, the radiating septa. I will specify 

 a few more peculiarities worthy of remark : — in some species 

 of the sub-genus Acasta, clefts are left, covered only by 

 membrane, on the lines of suture (PL 9, figs, la fid), between 

 the compartments, just above the basis ; and in other 

 species the basis is perforated by numerous membrane- 

 covered, minute orifices. In Platylepas, each compartment 

 has one deep inward fold (PL 17, fig. 1), somewhat analo- 

 gous to the three folds in Coronula ; this fold is produced 

 into an internal mid-rib, supporting and rendering convex 

 the membranous basis ; in this genus, also, the rostrum, 

 owing to its mid-rib, is generally thrust a little on one side, 

 and the shell thus rendered asymmetrical. In ChameesipJio 

 scutelliformis the shell is symmetrically perforated (PL 19, 

 fig. 4 a) by four apertures. Lastly, in Chthamalus Hembeli 

 and interteoctus, after a certain age, the basal edges of the 

 walls become inflected, and continue to grow inwards 

 till they entirely take the place of the true membranous 

 basis. 



Radius. — This term, as we have seen, is applied to that 

 side' of the compartment, the growth of which is modified, 

 by abutting against and overlapping the adjoining compart- 

 ment. Hence the structure of the radius is essentially the 

 same with that of the parietal portion of the compartment. 

 When best developed, as in Balanus tintinnabulum, the radius 

 consists of an outer and inner lamina, separated by denticu- 

 lated septa, extending in horizontal lines parallel to the 

 basis, and is consequently perforated by minute tubes or 



