too deep for the animal's body, and consequently the lower part of the deeply-concave basis has been filled up (Pl. 4, fig. 2 _a_) by thin, irregular, calcareous diaphragms. In elongated specimens, also, of _Balanus balanoides_, the shell sometimes appears to have grown too long for the animal's body; but in this case the membranous basis becomes extremely convex inwards; it still reaches the basal edges of the parietes all round, but in the middle it is raised high above the surface of attachment; yet sometimes threads of the cementing tissue depend from the middle part to the surface of attachment. In _Balanus terebratus_ (Pl. 8, fig. 2 _a_, 2 _b_), and in some species of Acasta, the basis is riddled, as previously stated, by numerous, minute, membrane-covered orifices. In _B. declivis_ the membranous basis is always extremely oblique, owing to the rostral end of the shell being twice as high as the carinal and opposite end. [Illustration: Fig. 11. Portion of edge of basis of _Balanus tintinnabulum_, _a_, _a_, outer lamina; _b_, _b_, inner or upper lamina; _c_, _c_, _c_, porose or cancellated radiating septa.] Regarding the very remarkable means by which the basis of sessile Cirripedes is cemented to the surface of attachment, it will be convenient to defer for a little the description, on account of its necessary length. _Structure of the Opercular Valves (Scuta and Terga)._ These are situated on each side of the slit or orifice leading into the sack; from their shape, their powers of movement, their separation by flexible membrane from the shell, to which they serve as a lid, they appear at first as if they constituted an element very distinct from the shell itself, but this is not the case. They are, together with the opercular membrane, as essentially as the whole of what is externally visible, a part of the modified carapace, of which they occupy the upper or posterior extremity: from tracing the metamorphoses, or even by comparison of a Balanus with Pollicipes, there can be no doubt of the truth of this conclusion. The opercular valves are four in number,--a pair of scuta and a pair of terga; but the latter in _Coronula diadema_ and _reginæ_, are either aborted or represented by a mere rudiment; and in Xenobalanus both scuta and terga are quite absent. In several cases, more especially in the genus Pyrgoma (Pl. 13, fig. 1 _b_), the scutum and tergum on each side are calcified together, so that sometimes not even a trace of the line of junction can be discovered. In most cases the scutum is firmly united, being articulated in a manner presently to be described, to the tergum; but in Coronula, Tubicinella (Pl. 17, fig. 3 _c_), and Platylepas, the ends of these valves are simply approximated.