The Top Shells and Dolphin Shells - Family Trochiae
THE TOP SHELLS AND DOLPHIN SHELLS - FAMILY TROCHIAE Shell top-shaped or conical, spiral, pearly within; oper culum thin, horny, spiral, usually circular, with a central nucleus; head with short, broad snout, long tentacles and simple eyes; radula well developed; three to five fleshy cirrhi on each side of mantle margin; one gill, the left; eggs laid on rocks or glued to seaweed in masses.
A large family including many genera of littoral and deep sea forms. They are chiefly vegetable feeders, living on algae. Chiefly tropical; the largest and handsomest species are distri buted in the Pacific and Indian oceans. Some are highly coloured in handsome patterns; all are pearly inside. A little shell, pearly and opalescent, from the East Indies, is gathered in quanti ties to be strung into necklaces. Large specimens of top shells are favourite ornaments for cabinets; the outer shell substance is usually ground off to reveal the pearl foundation of the closely wound coils. Some species are used in the manufacture of brace lets, buttons and pearl ornaments.
Genus TROCHUS, Linn.
Shell solid,
regularly conical, with high spire of many close whorls, which are usually angled and decorated with beading and bright colours in striking patterns; base of shell broad, flattened; aperture oblique, angled, with spiral operculum; columella twisted, its edges generally toothed or folded.
The genus is mostly confined to the Old World tropics.
The Great Top Shell (T. Niloticus, Linn.), the heaviest and largest of the top shells, is very striking in markings and coloration. The spire is sharp, the whorls white under zigzag radiating bands of red, violet or brown. The outer whorl flares decidedly, and has more and narrower stripes than the upper 213 The Top Shells and Dolphin Shells ones. The apical coils have nodules; the outer ones are smooth. The aperture is oblique; the columella has a spiral fold above and a strong tooth below. The umbilicus is shallow. These shells are from four to five inches in diameter and a trifle less in height.
Bracelets of pearl cut from these shells are highly prized by the South Sea Island women. If solid cross-sections cannot
be had, two or three pieces are fastened together with string. Five or six are worn on one arm.
Habitat.— Indian Ocean and Australian waters.
The Top Shell (T. acutangulus, Chemn.) of the same regions is a smaller shell; its apex is a 'very sharp point, and its white, sides are variously streaked with bright red. Rows of small beads adorn the whorls. The shells average two to three inches in height and somewhat less in diameter.
Habitat.— Eastern Seas.
The Toothed Top Shell (T. dentatus, Forsk.) has a heavy tall spire, its coils (about twelve) armed with large remotely set knobs, which stand out perpendicular to the surface. The colour is pale, with faint and fine markings of red. There is a broad band of green or blue surrounding the axis. This species is about three inches high and two and a half inches in diameter.
Habitat.— Red Sea, Persian Gulf.
The Common Top Shell of the Mediterranean coasts (T. ?iuphinus, Linn.) is regularly pyramidal, solid, glossy, with a strong rounded ridge spotted with brown at the base of each whorl. The ground colour ranges from brown to lilac, streaked in zigzag bands with brown. A full-grown specimen has a dozen coils. The animal is as vividly coloured as the shell. The promi nent head has a two-lobed flap between the long tentacles. The black eyes are on short stalks. Four pairs of cirrhi are thrust out sidewise from the mantle, as the creature crawls along the sea bottom, carrying the shell erect on its back, and the oper culum lying behind it on the extended foot. From low water mark to many fathoms depth this mollusk thrives, its food the seaweed. The shell measures somewhat over an inch in height and diameter of base. The lining is beautifully pearly.
A variety, conuloides, Lam., is found on the Atlantic coast of Europe. It shows great variability from almost smooth whorls to strong spiral ribbing, including all intermediate forms. In colouring there is little variation from the type species. The 214 The Top Shells and Dolphin Shells Atlantic forms are more depressed than those of the Mediter ranean.
