AXIS. The imaginary line, round which the whorls of a spiral shell revolve. The extremities of the axis are pointed out in fig. 379, by the letters, _a. a._ See "COLUMELLA." AZECA. Leach. _Fam._ Colimacea, Lam.--_Descr._ "Animal like Bulinus, with subcylindrical, rather obtuse shell, covered with a polished periostraca (epidermis); aperture pear-shaped, curved and pointed at the top; the margin thick, obtuse, united all round and toothed; the axis imperforated." Gray's edition of Turton's British Shells, page 189.--_Obs._ The Turbo Tridens of Montagu, upon which this genus is founded, resembles Bulinus lubricus in general form and character. Both these shells differ from the true Bulini in having the peritreme entire, and in being pellucid and glossy. Azeca differs from Bulinus lubricus in having three teeth in the aperture, two on the inner lip and one on the outer. Not seeing the necessity for creating a genus on grounds so slight, I have simply transcribed the description given above, leaving others to form their own conclusions as to the propriety of separating this shell from the genus Bulinus. Britain, Central and Southern Europe. Azeca Tridens, fig. 290. AZEMUS. Ranzani. CONIA, Leach. BACULITES. Lam. _Fam._ Orthocerata, Bl. Ammonacea, Lam.--_Descr._ Straight, conical, tubular, laterally compressed; chambers divided by very sinuous lobed septa, the last elongated; aperture elliptical; siphon dorsal.--_Obs._ This genus differs from Orthoceras in the same manner in which Ammonites differs from Nautilus, having its septa sinuated and branched. A Baculite might be described as a straight Ammonite. This genus is known only in a fossil state. It is found in the Cretaceous Limestone of Maëstricht and Valognes. Fig. 484. B. Faujasii.