THE THREE-ANGLED CLAMS - FAMILY TRIGONIIDAE. Shell equivalve, closed, three-angled, with beaks pointing backward; ligament external; hinge teeth few, diverging; interior pearly; pallial line simple; mantle open; foot long, bent; gills, two pairs. Marine.
Genus TRIGONIA, Brug.
Characters of the family. The distinctions of this small genus of Australian bivalves are, ( ) the brilliantly iridescent nacre that forms almost the whole of the shell substance; and, (2) the most closely interlocking hinge to be found among bivalves. The sheen of the pearl is sometimes golden, sometimes silvery; the ground colour is orange or deep rose-colour, according to the species. Diverging plates and grooves form comb-like hinge teeth before and behind the beaks. The outer surface of the shell is deeply scored with radiating grooves between rounded ridges, bearing nodose projections; the shell margin is crenu lated ; the gill margin frilled.
There are one hundred fossil species of this genus, found in the Devonian strata, all over the world. The three living species are confined to Australia.
The peculiar shell ornamentation found only in this genus of living mollusks occurs in fossil species of the fresh water Unios of Pliocene strata. This coincidence, with other structural simil
arities, leads Neumayr to believe that the great family of the Unio nidx is derived from Trigonia.
In the limestone quarries of Portland, England, the stone is often spoiled by the presence of "horse heads" — casts of Tri gonia, silicified nodules which have gradually filled and replaced the shells, which are so largely of pearl that they gradually disintegrate. Casts showing in detail the structure of the soft parts have been found.
379 The Three-angled Clams The activity of the animal is vouched for by Stutchbury, who, having dredged one in Sydney harbour, laid it on the gun wale of his boat, whence it leaped over a four-inch ledge and returned to its submarine abode. Strange migrations of these shell fish occur. All at once a region which has yielded abun dantly will become absolutely barren or them.
The Pearly Trigonia (T. mar garitacea, Lam.) is obliquely rounded in outline, and moderately convex, with ribs and nodules not very pronounced. It is the largest of living trigonias, and remarkable for its rosy purple pearl lining. Length, 2 inches.
Habitat.— Tasmania.
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