Seeing the state of the almost rudimentary shell, it is not difficult to imagine its total disappearance. Thinking of this, it occurred to me to doubt for a few minutes, whether Anelasma, described in my volume on the Lepadidæ, which presents many points of resemblance with [Xenobalanus],—viz., in colour, in the strength of the internal membrane of the sack, in the size of the orifice with its thin membranous margin, in the entire absence of opercular valves, and in epizoic habits,—might not in truth be a sessile cirripede, in an extremely altered condition; for the cirri of Anelasma are in so rudimentary a state, and the mouth in so modified a condition, that the internal characters by which pedunculated cirripedes can be distinguished from the [Balaninæ], though not from the [Chthamalinæ], are almost lost. But if Anelasma had belonged to either of the two sub-families of the [Balanidæ], the sack would have penetrated almost to the bottom of the peduncle; there would have been only a single layer of longitudinal muscles round the peduncle; and there would not have existed ovigerous fræna. I believe that Anelasma and [Xenobalanus] are only analogically connected, being no more related together by true affinity, than any other two genera in the Lepadidæ and [Balanidæ].
Sub-Family—CHTHAMALINÆ.
Shell with the rostrum having alæ, but without radii: rostro-lateral compartments without alæ on either side: parietes not porose.
Mouth with the labrum bullate; palpi hardly touching each other: third pair of cirri with the segments resembling those of the fourth pair.
The shell in this sub-family consists of four, six, or eight compartments, with the addition, in [Catophragmus], of several whorls of supplemental compartments or scales, like those forming the lower part of the capitulum in Pollicipes. The rostrum has alæ and no radii; in shape and size it resembles the carina. The rostro-lateral compartments are destitute of alæ; in all cases they overlap the adjoining compartments, and have radii (when such are developed) on both sides. In the genus, [Pachylasma], however, the shell must be looked to very young, in order to detect this normal structure, for soon the true rostrum and rostro-lateral compartments become blended together, making a compound rostrum, destitute, as in the [Balaninæ], of alæ, but furnished with radii. The parietes are never porose, nor furnished with regular symmetrical ribs (representing the longitudinal parietal septa) on their inner surfaces: sometimes, however, on the basal internal edges, there are some irregular depending points. In conformity with the simplicity of the parietes, the Radii (which are seldom much developed) have likewise a simple structure; and often are merely formed by the simple lateral growth of the parietes. The scuta and terga are articulated together more deeply than is usual in the [Balaninæ]; and the terga never have a long spur.
Looking to the animal’s body. The Labrum is always swollen and bullate, and though hollowed out, is never notched in the middle. The Palpi are rather small, and have not their tips nearly touching each other. The mandibles generally have their lower main teeth laterally single, though sometimes they are double: the inferior angle is always pectinated. The maxillæ are always notched under the upper pair of great spines. Of the Cirri, the third pair much more closely resembles, in external structure and powers of movement, the fourth than the second pair, though sometimes a few of the basal segments, especially on the anterior ramus, are thickly covered with bristles, as on the segments of the second pair. In three species belonging to two genera, there are caudal appendages, resembling those generally present amongst the Lepadidæ, but never found in the [Balaninæ]. The Branchiæ are generally smaller than in the [Balaninæ]; and are sometimes quite rudimentary.
In the introductory description (p. [152] and [176]) of the Family [Balanidæ], I have already discussed the relations of the [Chthamalinæ] to the [Balaninæ], and of the several genera to each other, so that I need not here re-enter on the subject.