ANTERIOR. In Bivalves is the side on which the head, or part analogous to the head of the animal lies; it is known in the shell by the umbones, which if turned at all, are turned towards that part. If there be a sinus in the impression of the mantle, it is always on the posterior part of the shell. If the ligament be placed only on one side of the umbones, it is only on the posterior side. The anterior of a _spiral univalve_ is that part of the outer lip which is at the greatest distance from the apex. Of a _symmetrical_ conical univalve such as Patella, it is that part where the head of the animal lies, indicated by the interruption of the muscular impression. Of _cirripedes_, that part where the ciliæ protrude are anterior; of _Brachiopoda_, that part which is farthest from the umbones and which corresponds with the ventral margin in other Bivalves. The anterior of _symmetrical, convolute univalves_, is the outer or dorsal part of the aperture, or that part which is farthest from the spire. Lamarck and other Conchological writers have occasioned much confusion by their errors on this subject; describing the same part of a shell at one time anterior, at another posterior; but generally the reverse of the above arrangement, which is founded upon the natural position of the animal, and generally adopted. The anterior will be indicated by the letter _a_, in figs. 119, 421, 229, 34, 202. ANTIGONA. Schum. A genus composed of VENUS cancellata, Lam. (fig. 119.) and similar species. ANTIQUATED. This word, signifying _out of date_, is occasionally used to express that species of composition which constantly occurs in shells, by each fresh deposit or layer of calcareous matter, forming a new margin, which being replaced by its successor, is no longer used as the margin, and is consequently said to be out of date. APEX. This term does not apply to the natural position of a shell, but is used in a mathematical sense, to indicate the nucleus or first formed part; which may be considered as the point of the spiral cone. From this point, the shell enlarging rapidly or slowly as it descends, takes a spiral, arched, straight, oblique, convolute, or irregularly spiral course. The apex will be indicated by the letter _a_, in fig. 282 and 466.