_Fossil Species._--Having already given, under the Family, some account of the geological history of sessile cirripedes, short as it is, I here only allude to the subject in order to state my conviction that species cannot be satisfactorily distinguished in a fossil state, and rarely in a recent state, without an examination of the opercular valves. Nothing, indeed, could have been easier than to have affixed names to many groups of specimens, having different aspects, but to feel sure that these were really distinct species requires better evidence than can be afforded by the shell, without the operculum. No doubt, in some of the smaller sections of the genus--for instance, in that characterised by a membranous basis--it would have been possible to have distinguished some or several fossil species; but such have not as yet been found. When the specimens are much fossilised, it is, indeed, difficult to make out the primary points of structure--namely, whether the parietes, radii, and basis are porose: to do this it is sometimes necessary to rub down, polish, and carefully examine, a transverse section of a piece of the shell. _Sections of the Genus._ [A] _Parietes, and basis, and radii permeated by pores._ [B] _Parietes and basis sometimes permeated by pores, sometimes not; radii not permeated by pores; shell elongated in its rostro-carinal axis; basis boat-shaped, attached to Gorgoniæ and Milleporæ._ [C] _Parietes and basis permeated by pores; radii not permeated by pores._ [D] _Parietes permeated by pores. Basis and radii not permeated by pores._ [E] _Basis membranous._ [F] _Parietes and radii not permeated by pores. Basis sometimes permeated by pores, sometimes not, sometimes excessively thin and hardly distinguishable._[86] [86] The following species might, owing to variation, or to the obscurity of their structure, without care, be classed in the wrong sections:--in A, _Balanus Ajax_, from living attached to Milleporæ, is sometimes elongated in its rostro-carinal axis, and from having its radii only finely porose, it might be classed in B: on the other hand, _Bal. stultus_ is sometimes but little elongated, and the basis hardly boat-formed, and hence might be classed in A. The distinction between all the species in (B) and the sub-genus Acasta is artificial. In sections C and D, I have seen one specimen of _B. spongicola_ with a solid basis, and very young specimens of _B. improvisus_ are thus characterised, and therefore these species are liable to be placed in the wrong section, D: _Bal. nubilus_, also, has part of its base non-porose, and therefore likewise might be placed in D: on the other hand, the circumference of the basis in _B. patellaris_ is often porose, and hence this species, which belongs to D, might be placed in C.