STRUCTURE OF SHELL. 49 



membrane is formed, and is attached to the next lower zone 

 of the sheath ; the old membrane being cast off, but a 

 circular slip of it is left, investing the last zone. Hence 

 the whole upper part of the sheath above the opercular 

 membrane, comes to be thus invested ; and is marked by 

 circular lines, one above the other, caused by the successive 

 exuviations. This investing membrane often supports rows 

 of minute bristles, directed upwards. Generally, a film of 

 shell is deposited, at the period of the formation of each 

 new opercular membrane, on that part of the sheath which 

 lies immediately beneath. This innermost film or thin 

 layer of shell, on the lines of suture between the compart- 

 ments, breaks joint, at least in some cases, with the under- 

 lying shelly layers, — that is, the suture in this last-formed 

 film does not lie exactly over the suture in the subjacent 

 layers of the sheath. In Tubicinella, the sheath extends 

 down close to the basis ; and what is unique in this one 

 genus, the opercular membrane, gradually thinning out 

 downwards, closely adheres to the whole inner surface of 

 the shell. In Tubicinella and in Xenobalanus (PI. 17, 

 fig. 4 b), the sheath separates easily into separate successive 

 rims of shell ; and this structure evidently is for the sake 

 of facilitating the breakage of the upper end of the shell, 

 which, as we shall presently see, is necessary to allow of the 

 increase in size of its orifice. 



Basis. — This, in several genera and species, is composed 

 of simple membrane, and consists of successive, concentric, 

 circular slips, added round the outside, at each period of 

 growth. In some species of Tetraclita and Balanus the 

 basis is calcareous, but diaphanous, very thin, smooth, or 

 somewhat granulated. In other cases it consists of a single 

 calcareous lamina, either smooth, or with ridges radiating from 

 its centre ; it is formed of two laminae, (as is most usual in 

 Balanus,) separated by radiating septa. These septa, as well 

 as the radiating ridges in the case of the single lamina, are 

 homologous with the longitudinal septa of the parietes. 

 The denticulated ends of the latter enter the mouths of the 

 tubes formed by the radiating septa of the basis : threads of 

 corium pass between the denticuli of the parietal septa, and 

 thus enter the basal tubes. The ends of these threads of 



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