54 BALAiMD^E 



sometimes feebly developed, for the attachment of the tergal 

 depressor muscle. But in Chelonobia, Coronula, Tubicinella, 

 Platylepas, and in some other cases, there are no crests. 

 The crests, when well developed, are furnished with rectan- 

 gular sub-crests or dcnticuli on both sides ; in fact they 

 resemble, and are probably homologous with, the denticulated 

 ribs or septa in the parietes, radii, and basis. Altogether 

 the scuta and terga are attached, as far as muscles are con- 

 cerned, to the shell and sack, by three longitudinal pairs. 



Growth, of the whole Shell, audits Microscopical Structure. 

 — The opercular valves are added to along their basal 

 margins alone;* the animal's body, together with the 

 several muscles, becoming attached at each period of growth 

 lower and lower down to the valves ; this no doubt is 

 effected by the absorption of the upper surfaces of the 

 muscles, and the formation of new fasciae on their lower 

 surfaces. The opercular membrane, which, though thin 

 and flexible, forms part of the general outer surface of 

 the animal as much as does any portion of the rigid shell, 

 with which indeed it is strictly homologous, is periodically 

 moulted, together with the integuments of the whole included 

 animal. The new opercular membrane is of course each 

 time formed a little larger than the old one. In Coronula 

 and Tubicinella, however, several successive opercular 

 membranes are preserved one over the other, and the out- 

 side membrane gradually disintegrates ; in these cases the 

 undermost opercular membrane is formed wrinkled and 

 considerably too large, so as to allow of being stretched, 

 before it is finally cast off. In Tubicinella, the opercular 

 membrane runs down, adhering to the inner surface of the 

 shell, to nearly the basis, and hence during the diametric 

 growth of the shell, it is longitudinally split, and is repaired 

 by slips of new membrane, which resemble the radii in 

 form and in direction of the lines of growth. 



The basis is added to only round the circumference, and 



* In some species of Pyrgoma, the ledge {limbics occluclcns) which is added 

 along the occludent margin of both scuta and terga, and in some species of 

 Balanus a narrow rim, or slight protuberance which is added along the carina] 

 margin of the terga, offer unimportant exceptions to the rule, that the oper- 

 cular valves grow only at their basal margins. 



