In the annexed woodcut (fig. 1), of the rostrum of _Balanus Hameri_, the downward growth and the lateral growth on both sides is plain. The modified sides (_rr_) for convenience sake, have been called the _radii_; they invariably overlap the adjoining compartments. The middle part (_p_), has been called the wall, or parietal portion: in the specimen figured, the walls and radii are distinctly separated, but in some cases, especially amongst the Chthamalinæ, the lines of growth are absolutely continuous from one to the other. In fig. 2 of a Lateral compartment of the same Balanus, we have the same essential structure; but the left side (_a_) is more protuberant, and is hollowed out in its lower half; it is, also, more distinctly separated from the parietal portion: this side has for convenience been called the _ala_; it is invariably overlapped by the adjoining compartment: in some few cases, as in Pachylasma, the ala is not hollowed out in its lower part, and from being added to in a straight line along its whole edge, with the lines of growth continuous with those on the wall, it differs hardly at all in appearance from a radius. Lastly, in fig. 3 of the carina, or compartment facing the rostrum, we have alæ (_aa_) on both sides; these being, as in all cases, overlapped by the adjoining compartments. [Illustration: Fig. 1. Fig. 2. Fig. 3. _p_, _p_, Parietes; _r_, _r_, Radii; _a_, _a_, Alæ. Fig. 1, Rostrum with two radii, serving in the Chthamalinæ for rostro-lateral compartments. Fig. 2, always serving for lateral and carino-lateral compartments. Fig. 3, Carina, serving in the Chthamalinæ, also, as a rostrum.] Now, the compartments in the shell of every sessile Cirripede, are without exception constructed on the above three simple patterns. In number, they are 8, 6 or 4, or all confluent together.