APERTURE or MOUTH. The entrance to the spiral cavity of univalve shells. The parts of the aperture are separately described, as follows: The inner lip or labium is that part which lies over the preceding whorl of the shell. It terminates anteriorly, or towards the lower part in what is termed the columella, so called because it forms a kind of axis on which the volutions turn. The outer lip, sometimes called the labrum, is on the opposite side, or the farthest from the axis. If the edges of the inner and outer lips unite all round, they are described as composing the peritrême. In fig. 318, the aperture is marked by the letter a. APHRODITA. Lea. (from [Greek: Aphroditê], Greek name of Venus.) A genus composed of CARDIUM Groenlandicum, Auct. fig. 123*, and other similar species of Cardium, the teeth of which are either wholly wanting, or very indistinct. Northern Ocean. APICIAL. Belonging to the apex. The apicial extremity of the aperture of a univalve shell, is that which is nearest to the apex of the spire. APICULUM. Humph. TROCHUS, Lam. APLEUROTIS. Rafinesque. A genus unfigured and imperfectly described as differing in some respects from Terebratula and other Brachiopodæ. APLEXUS. Fleming. A genus composed of PHYSA Hypnorum, Drap. &c. and described as having the inner lip simple, and not spread over the body whorl. APLODON. Rafinesque. A genus proposed to be established at the expense of the genus HELIX, but upon what grounds does not appear from the imperfect description which is unaccompanied by a figure. APLUSTRE. Schum. A genus formed for the reception of those species of BULLA which have the spire uncovered. _Ex._ Bulla Aplustre, (_aplustre_, a flag.) Auct. fig. 289. APLYSIA. Linn. (_a_, without; [Greek: Pluô], to wash.) _Fam._ Laplysiens, Lam. Aplysiana, Bl.--_Descr._ Horny, transparent, clypeiform, or shield-shaped, placed horizontally on the back of the animal, with its convex side uppermost; apex slightly incurved.--_Obs._ The animal producing this shell has derived its name from the purplish liquor which it exudes, when disturbed. In contour, it has been fancied to present a certain likeness to a hare crouching, and on this account was called _Lepus marinus_, or sea hare, by the ancients. The shell bears a strong resemblance to Dolabella, which, however, is much thicker, and more testaceous. The species are found in the Mediterranean, European, and West Indian Seas. A. Petersoni, fig. 254.