TFBICINELLA TRACK EALIS. 433 



gated shell, and terminates in a slight shoulder : it is divided as in 

 common Balanidse, and differently from in Coronnla, into zones of 

 growth, but these are very broad ; at the upper end of the shell, which, 

 as will hereafter be explained, is always breaking away, the sheath 

 readily yields along the oblique planes, which separate the zones of 

 growth and dip outwards: a similar but less strongly marked structure 

 occurs in Platylepas, and in no other genus. We shall presently see 

 that the sheath presents a much more anomalous character, in being 

 lined down to its basal edge by the innermost and last-formed layers of 

 the opercular membrane. 



Radii. — The radii are narrow. The belts which surround the shell 

 are prolonged, with slightly diminished prominence, across the radii, 

 their formation being simply due to the radii being here thicker than in 

 other parts. It can be seen more plainly in Tubicinella, than in other 

 Balanidae, that the membrane externally investing the shell, splits along 

 the radii during the diametric growth of the shell, and is continually 

 repaired and added to along these lines by new longitudinal slips of 

 membrane. The radius consists (as usual) of an inner and outer lamina, 

 which latter does not extend quite to the line of suture — a slightly gaping 

 fissure being thus left. The two laminae are connected by septa, which 

 are not denticulated, but near the outer lamina bi- or tri-furcate, and 

 the ends of the branches thus formed spread out, forming a sort of outer 

 scalloped lamina, in advance of the true outer lamina. The fine 

 threads of corium running between these septa, do not spring, as in all 

 common cirripedes, from a fillet of corium occupying the actual suture, 

 but from two nearly circular threads of corium occupying two tubes, 

 which run along the line of junction between the radius and the com- 

 partment whence it springs. In Coronula alone we have a nearly 

 similar structure ; for the fine threads of corium occupying the proper 

 radius, spring from a single very minute tube (PI. 16, fig. 7, d'), 

 occupying the same position with the two tubes in Tubicinella. I may 

 further add, that the structure of the proper radius in Coronula is 

 precisely the same as here just described, but being on so very minute 

 a scale, I did not there describe it so carefully as I have here done. 



Alee : these^are only remarkable from their extreme thinness ; for they 

 are not thicker than the inner lamina of the radius. Their sutural 

 edges are quite smooth. Forming part of the sheath, they extend 

 down close to the basis of the shell ; where, instead of, as in general, 

 ending abruptly in a rectangular shoulder, they slope offinto their own 

 compartment. 



Basis. — The basal membrane is complicated, owing to the shell, when 

 full-grown, barely, or not at all, increasing in diameter, and, in con- 

 sequence, membrane after membrane, each with its own cement-ducts 

 attached to it, are thrown down one nearly over the other. In the 

 Introduction (p. 143 — PI. 28, fig. 3), I have fully described the 

 cementing apparatus, which is very curious from one of the ducts 

 always having a loop with two spurs projecting from it. The basal 

 membrane does not equal in diameter the base of the shell, for the mem- 

 brane externally covering the walls is inflected inwards all round for 

 a considerable width, and is then united to the basal membrane : in 



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