Technical expressions are unavoidable when describing the details of a seashell. I have compiled a list of terms associated with seashells for your reference. They are arranged in an A-Z sequence for your easy search and reading. 

Aperture - opening at the front end of gastropod shell and a tusk shell.

Apex - starting point of shell growth, the 'top' of a gastropod spire.

Apophysis - a projecting spur inside a bivalve shell below the umbo, the site of a muscle attachment.

Axis - an imaginary line around which the whorls revolve, drawn from the anterior to the apex of gastropods.

Bivalve - a mollusc with s shell formed of two pieces.

Body whorl - the largest section of a gastropod, this encolses the soft parts.

Byssus - a bunch of silky threads anchoring some bivalves to solid object.

Calcareous - made of calcium carbonate, chalky.

Callus - a  thick or thin layer of shelly substance, smooth and glossy, usually transparent.

Canal - a channel at the top or bottom of the aperture to accommodate the siphon of a gastropod.

Cancellation - a sculptured area of lines crossing others at right angles, in a lattice-like effect; also referred to as reticulation.

Cardinal tooth - projection on the hinge plate below the umbo.

Chiton - a mollusc with a foot, a head bearing tentacles and eight shelly plates held together by a girdle.

Columella - the spirally-twisting pillar surrounding the axis of a gastropods. 

Concentric- following the direction of growth lines, mostly in bivalves.

Ear - an extension of the hinge region of a bivalve shell, as in scallops.

Escutcheon - a depression found behind the umbones of  a bivalves shell, often compassing an extenal ligament.

Family - a unit comprising of two or more closely related species belonging to one genus or two or more genera, subordinate to a superfamily.

Fluted - scalloped or arched.

Foot - the fleshy sole that a gastropod creeps or glides upon, or by which it adheres.

Gastropod - any of various mollusks of the class Gastropoda, such as the snail, slug, cowrie, or limpet, characteristically having a single, usually coiled shell or no shell at all, a ventral muscular foot for locomotion, and eyes and feelers located on a distinct head.

Genus (Genera) - a unit comprising one or more species, subordinate to a family. 

Girdle - the muscular ribbon surrounding and binding together a chiton's shelly plates.

Globose - rounded or almost spherical.

Granulose - tending to be inflated, like a ball.

Growth line - a fine or coarse, raised line defining a temporary pause in the growth of the shell. Otherwise knowth as ridge. 

Hinge - inner edge of the valve of a bivalve shell, usually joined by a ligament to the opposite valve and usually bearing teeth that interlock with teeth in that valve.

Impressed - indented or sunken, a term usually applied to the suture.

Incised - finely scratched.

Incurved - turned inwards.

Keel - a raised, often sharp ridge.

Ligament - the elastic, corneous structure joining the valves of a bivalves.

Lip - the inside or outside edge of the aperture of a gastropod shell.

Lunule - a heart shaped depression in front of the umbones of a bivalves shell.

Mantle - a fleshy lobe which secretes the mollusc's shell and lines the inside of the shell wall.

Margin - the shell edge.

Mollusc (Mollusks) - A soft body, legless, invertebrate animal which usually secretes a calcareous shell.

Mother-of-pearl - a shelly layer or the inner lining of a shell, displaying a pearly surface.

Muscle scar - the impression inside the valve of a bivalve shell made by a muscle holding the valves together.

Nympth - the narrow ledge, on the hinge behind the umbo, to which the external ligament is attached.

Obsolete - ceasing to exist, no longer present.

Operculum - grown on the foot of many species of gastropod, this is an oval or rounded structure which seals the aperture when the animal withdraws into its shell.

Ornament - raised or depressed features on the shell surface.

Ovate - oval; egg-shaped.

Pallial line - an impressed line inside the valve of a bivalve shell parallel to the margin, which marks the position of the mantle edge.

Parietal area/wall - occasionally referred to as the inner lip, this is the area in gastropods that lies opposite the outer lip and above the columella.

Radial - ray-like ornamentation or sculpturing, diverging from the umbones of bivalves; the direction of the raised or depressed ornamental feature of a bivalves, running form the umbo towards the edge of the valve.

Range - the total distribution of a species.

Recurved - curving upwards or downwards.

Rib - a continuuous elevation of the shell surface.

Ridges - see growth lines.

Scale - raised, sharp edged, ornamental feature, sometimes fluted.

Serrated - edge of a shell that exhibits a series of grooves or points.

Shelf - a thin plate obsuring the aperture of some species of gastropod shells.

Shell tube - hollow tube occupied by most gastropods, all tusk shells and few bivalves.

Shoulder - the angulation of a whorl at or just below the suture.

Sinuous - greatly waved.

Siphon - a rectractable fleshy tube of gastropods and bivalves, used for various purposes, including feeding and the explusion of waste.

Siphonal canal - otherwise known as notch; the tube or gutter at the front end of the aperture, to hold the front siphon of a gastropod.

Slit - a shallowor deep cut in the shell margin of some gastropods and in the umbones of some bivalves.

Species - a group of beings sharing virtually identical characteristics; it is subordinate to and contained within a genus.

Spine - a blunt or sharp projection, often found in murex.

Spiral - on gastopod shells and tusk shells, going in a transverse direction.

Spire - the coiled part of a gastropod shell apart from the body whorl.

Stria (striae) - fine raised or grooved line on the surface of a shell, sometimes an indication of growth stage.

Suture - the junction of two whorls, it is often depressed or indented.

tooth (teeth) - pointed or blunt structure(s) inside the outer or inner lip of a gastropod shells, and on the hinges of bivalves shells.

Umbilicus - the lower open axis around which the whorls of a gastropods are coiled.

Umbo (Umbones) - the earliest-formed part of a bivalve shell's valve.

Valve - one piece of a bivalve shell or of a chiton.

Whorl - one complete turn of the tube of a gastropod shell about its imaginary axis.

Wonderful Selection of BEAUTIFUL Seashells

My other Review titled "Gastropods Seashell Family Guide (with pics)"