Parietes and basis permeated by pores; radii not permeated by pores.
Parietes permeated by pores. Basis and radii not permeated by pores.
Basis membranous.
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.