438 BALANID.E. 



had even a more tapering form than that represented by the whole and 

 dotted lines in the two figures, and would have exceeded six inches in 

 length ! this of course being also the length of the whole shell. The 

 young Tubicinella, of which 3 b is a compartment, was imbedded in the 

 whale's skin nearly up to the level of its operculum; if it had lived, 

 it would no doubt have grown to the length just specified, via., above 

 six inches, but as all the growth is at the lower end, the bottom of the 

 shell, it might be thought, would necessarily have become buried in 

 the whale's skin to this same depth ; and the summit of the shell, on 

 this same view, would have been buried to a depth by as much less as the 

 height or length of the old shell itself, namely, by about one inch and a 

 half less than the six inches. As far as I can judge from an examination 

 of several large groups of full-grown specimens, preserved in their im- 

 bedded condition, the summits of the shells seem alwavs to lie a little 

 beneath the surrounding level of the whale's skin, but not nearly to the 

 extent here just inferred. Nor can I believe that the epidermis of the 

 whale had ceased being formed under these specimens, whilst it had gone 

 on being formed all round them, to the thickness of between four and five 

 inches, and that it had subsequently disintegrated to this same thickness, 

 — which processes would account for the summit of the shell being still 

 on nearly a level with the surface of the whale. The view which seems 

 to me most probable, is, that the rapid downward growth of the shell, 

 besides indenting the whale's skin, at the same time slowly pushes the 

 whole shell out of the skin, and thus continually exposes the summit to 

 the wear and breakage which seems to be necessary for its existence. 

 On this view, the very peculiar form of Tubicinella, which is retained 

 during life, namely, the slightly greater width at top than at bottom, 

 is beautifully explained, viz., for the sake of facilitating the protrusion 

 of the shell ; for the ordinary conical shape of sessile cirripedes, with 

 the apex upwards, would have rendered the pushing out of an imbedded 

 shell almost impossible; on the other hand, we can see that the like- 

 wise very peculiar, concentric, prominent belts may be necessary to 

 prevent too easy protrusion. 



Fossil Species. — I do not believe that this genus has hitherto been 

 found fossil. The Tubicinella maxima of Ch. Morren, said to have been 

 found (see Bronn, Index Palseontologicus) in the chalk of Belgium, I 

 have good reason to believe does not really belong to this genus. 



1 1. Genus — Xenobalakus. PI. 17, fig. 4 a — 4 c. 



Xenobalanus. Steenstrup. Videnskabelige Meddelelser. Aaret, 



1851. 

 Siphonicella (sine descript.) Darwin. Monograph on the Lepa- 



didse, p. 156 (1852). 



Shell almost rudimentary, star-formed, composed of six 

 compartments, with a long peduncle-formed body rising 

 from the middle : opercular valves none. 



