Under the head of _B. tintinnabulum_ I have alluded to the great variation of _B. amphitrite_, which consists not only in a vast diversity in the colouring and in the general aspect, but likewise in the degree of obliquity of the summits of the radii, in the form of the terga, and slightly in that of the scuta. In order to show that it has not been from indolence that I have put so many forms together, I may state that I had already named and fully described in detail eight of the following forms as species, when I became finally convinced that they were only varieties: it would require at least thirty figures, which I have not the power to give, fully to illustrate the transitional forms. As with _B. tintinnabulum_, the deception is wonderfully enhanced by whole groups of specimens from the same locality exactly resembling each other, and sometimes differing from other groups attached to the very same object. If a person were to get together only some fifty or sixty specimens from only half a dozen different localities, he would almost certainly come to the same conclusion, as I at first did, that several of the varieties are true species; but when he gets several hundred specimens from all quarters of the globe, he will find, to his trouble and vexation, that character after character fails and blends away by insensible degrees, and he will be led, as the more prudent course, to include, as I have done, and I hope rightly, all under one specific name. I have experienced more doubt regarding the last variety, _cirratus_, than on any other, on account of its peculiar colouring, and from the basis being often irregularly cup-formed. Under _B. concavus_ I have remarked how closely some of its varieties approach to _B. amphitrite_, and it is to this last variety that they approach; almost the only difference being that the scuta in _B. concavus_ are longitudinally striated. Yet some of the varieties of the two species are so distinct that it would be puerile to class them together. I will only add, that after studying such varying forms as _B. tintinnabulum_ and _amphitrite_ it is difficult to avoid, in utter despair, doubting whether there be such a thing as a distinct species, or at least more than half a dozen distinct species, in the whole genus Balanus. As with _B. tintinnabulum_, I will first give a full description of the more common forms, alluding only to each less frequent variation, and then separately describe briefly the more marked varieties.