[129] I am indebted to Dr. J. E. Gray for calling my attention to this subject, and explaining to me several points.
Some curious results follow from the peculiar growth of [Tubicinella] just described. At Plate [17], fig. [3 b], we have a careful drawing of a lateral compartment, together with its radius, (which latter does not here concern us), taken from a shell .2 of an inch in diameter. The two protracted dotted lines show the form which this compartment would have assumed, if it had continued growing downwards at the same rate of increase in width as hitherto. But the increase in width always seems to become less and less as the shell grows older; hence the dotted lines, representing the wall after long-continued growth, ought to have been drawn diverging or widening still more slowly than they do. The lateral compartment in fig. [3 a] is the exact size of the compartment of a large specimen nearly one inch in diameter; in this specimen the parietes, far from increasing in width downwards, had commenced, as is represented, decreasing. Compartments of all intermediate sizes between those figured at [3 a] and [3 b] can easily be shown in different specimens. From these facts we may safely infer, that if the whole growth of the compartment [3 a] had been preserved, instead of its upper end having been continually chipped away, it would have 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, viz., 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 imbedded condition, the summits of the shells seem always 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 likewise 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 Palæontologicus) in the chalk of Belgium, I have good reason to believe does not really belong to this genus.
11. Genus—XENOBALANUS. Pl. [17], fig. [4 a]-[4 c].
XENOBALANUS. Steenstrup. Videnskabelige Meddelelser. Aaret, 1851.
SIPHONICELLA (sine descript.) Darwin. Monograph on the Lepadidæ, 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.
Hab.—North Atlantic Ocean, attached to Porpoises; Mus. R. T. Lowe, Steenstrup.