When first examining the parasites of _S. rostratum_, _S. Peronii_, and _S. villosum_, before the weight of the cumulative evidence had struck me, and noting their apparent state of immaturity, it occurred to me that possibly they were the young of their respective species, in their normal state of development, attached to old individuals, as may often be seen in Lepas; this, however, would be a surprising fact, considering that _S. rostratum_ and _S. Peronii_ are ordinarily attached, in a certain definite position, to horny corallines, and considering that the exact points of attachment in these three parasites, (of which I have seen no other instance amongst common Cirripedes,) namely, between the scuta, would inevitably cause their early destruction, either directly or indirectly, by their living supports being destroyed. Nevertheless, I carefully examined a young specimen of _S. rostratum_ only thrice as large as the parasite; and not having very young specimens of _S. Peronii_ and _villosum_, I procured the young of closely-allied forms, namely, of _S. vulgare_, (with a capitulum only 4/100th of an inch in length,) and of _Pollicipes polymerus_, (with a capitulum of less size than that of one of the parasites,) and there was not the least sign of anything abnormal in the development of the valves. In _S. vulgare_, at a period when the calcified scuta could have been only 1/100th of an inch in length, (and therefore considerably less than the scuta in the parasites,) the upper latera must have been as much as 4/1000ths of an inch in length, and the valves of the lower whorl certainly distinguishable.