58 BALANID^l. 



prior to the period of moulting and growth, I found the 

 unaltered corium so charged, as to effervesce, with carbo- 

 nate of lime, either in a state of dissolution, or in granules 

 too minute to be visible under the highest powers. 



The sutures between the several compartments and the 

 basis are covered by thin membrane, which is continually 

 splitting during the growth of the opposed edges of the 

 underlying shell ; but previously to each splitting, a new 

 slip of membrane is, I believe, already formed under the old 

 one ; so that the corium is not even momentarily exposed. 

 Owing to this manner of growth, the slips of membrane con- 

 sist of successive rims united together ; in most cases, these 

 soon become abraded from the older parts of the shell, but are 

 sometimes preserved. The last-formed slip of membrane 

 over a suture is homologous with the opercular mem- 

 brane ; and both are strictly analogous with the ring of 

 flexible membrane, forming the joint of the leg of a crab. 

 In the latter case, the flexible membrane and hardened 

 crust are both moulted together : in the opercular mem- 

 brane, there is a double line of splitting, one close round 

 the opercular valves, and the other at the basal edge of the 

 sheath, and the intermediate portion is moulted, but with a 

 zone of membrane left adherent to the non-moulted valves 

 and sheath : lastly, in the slips of membrane covering the 

 sutures, there is only a single line of splitting, and no por- 

 tion, I believe, is moulted ; the rims of membrane on each 

 side remaining adherent on the compartments and basis, 

 until worn away. 



The opercular membrane, when closely examined, ex- 

 hibits no structure, except that it can sometimes be plainly 

 seen to be composed of successive, numerous, excessively 

 thin laminae. Occasionally, however, it presents the false 

 appearance of being permeated by parallel and anastomosing 

 vessels : this appearance is due to one or more of the com- 

 ponent laminae having been wrinkled before a succeeding 

 lamina was thrown down and attached to its under side. 

 If a small piece of an opercular valve of Tubicinella, with 

 the opercular membrane adhering to it, and with the 

 corium under both, be dissolved in acid, it may be clearly 

 made out that the corium under the valve has gone on 



