The Megatrema semicostata of G. B. Sowerby, junr. (Conch. Manual, fig. 33, 1839), is not described, and is very indifferently figured without the opercular valves, and therefore can never be recognised.
There is an admirable figure of a [Pyrgoma], without any specific name, in the great ‘Description d’Egypte,’ but from the want of some details, I cannot positively recognise the species; I am inclined to believe that it is the [P. dentatum] of this work.
M. Chenu, in his grand Illustrations Conchyliologiques, has given most beautiful figures of several species of [Pyrgoma], and of [Creusia], but unfortunately, from the opercular valves not having been figured, I find it impossible to recognise them. The new species are Pyrgoma stellata (on which I have appended a note under [P. conjugatum]), and P. spongiarum and P. corymbosa of Valenciennes.
6. Sub-Genus—CREUSIA. Pl. [13], [14].
CREUSIA. Leach. Journal de Physique, tom. 85, July, 1817.
Compartments four, furnished with radii; basis cup-formed: attached to corals.
Distribution, imbedded in corals throughout the tropical seas.
[Creusia] is closely allied to [Pyrgoma]; and had not this genus already been adopted by several authors, I should not, I think, myself have formed it; though, no doubt, it harmonises well with some of the other genera of the family, which are perhaps all too intimately related. [Creusia] differs from [Pyrgoma] only in the shell being separated by sutures, into four compartments, with well developed radii: in other respects, such as in general habit, in the cup-formed imbedded basis (not permeated by pores), in the opercular valves, in the characters derived from the mouth and cirri, there are no generic differences. This affinity is more particularly evident when [Creusia] is compared with the first few species of [Pyrgoma]: indeed, for a short time, I was inclined to consider var. 10 of [Creusia] as identical with [Pyrgoma conjugatum]. With respect to the species of [Creusia], I confess I have been much perplexed in determining whether there be only one, or half a dozen. The latter conclusion would almost certainly be arrived at if only a few specimens were examined; and it might naturally be thought that some of the species were extremely well marked; the difficulty of drawing any line between varieties and species, begins only when some hundreds of specimens, from various parts of the world, are disarticulated, cleaned, and carefully examined. [Creusia], in this respect, offers a striking contrast with [Pyrgoma], in which nearly all the species are strongly characterised. The shell differs very little in the several varieties or species of [Creusia]; and the most marked difference, namely, whether the walls are permeated by irregular pores or not, seems certainly quite variable. It is in the opercular valves, which in other genera offer by far the most reliable character, that we encounter the chief cause of perplexity; for the characters thus derived, though at first appearing very distinct, blend into each other, and are not accompanied by any well marked differences in the shell. Only a few of my specimens have any habitat; but the geographical range, as far as it does go, throws no light on the question which forms to regard as species and which as varieties. As is generally the case with cirripedes, the variations are local, so that the greater number of specimens imbedded in the same coral resemble each other. Under these circumstances I have thought it best, after repeated examinations of a very large suite of specimens, to describe separately each variety, without attaching any name to it; but I will first make a few general remarks on the structure of the shell. If I do not thus throw much light on the subject, I shall at least not burden it with error. I believe that the species will be definitely made out only by persons resident in the coral-bearing zones. I have given copious illustrations of the opercular valves; for, if my view be correct, this genus offers a curious and striking case of variation; if, on the other hand, I am wrong, the drawings, I hope, will aid others in coming to a more correct conclusion.