Cucullaea concamerata (Martini, 1777) (syn. C. granulosa Jonas 1846) possesses a posterior buttress in each valve. The function of this structure has been investigated and it is concluded that it reduces the length of (thereby assisting valve alignment), and the effect of shear upon, in particular, the ‘catch’ component of the posterior adductor muscle. The shell of Cucullaea is inflated and the posterior face is antero-posteriorly flattened, so that this region lies flush with the surface of the sand. These are adaptions to this species’ mode of life, conferring stability in an unstable niche. The buttress was a necessary co-requisite of these modifications. Buttresses of Cucullaea and members of the Laternulidae are compared and shown to be of quite different function; the buttress of Cucullaea is more meaningfully compared with the septum of members of the Dreissenacea which fulfils very similar functions.