The royal natural history . uncuhis inform, but the shells are more com-pressed and clothed with a fibrousperiostracum, and the animalspins a byssus. Several of thespecies have been dredged atenormous depths in the genus Trigonia, representedby about half a dozen speciesoccurring on the shores of Australia, is all that now remains of the large family Trigon i!<l<i\ of which severalother genera, with a very large number of species, occur fossil in the Secondaryand Tertiary rocks. The valves of Trigonia are beautifully pearly within, equal,radiately ribbed, with an external li
The royal natural history . uncuhis inform, but the shells are more com-pressed and clothed with a fibrousperiostracum, and the animalspins a byssus. Several of thespecies have been dredged atenormous depths in the genus Trigonia, representedby about half a dozen speciesoccurring on the shores of Australia, is all that now remains of the large family Trigon i!<l<i\ of which severalother genera, with a very large number of species, occur fossil in the Secondaryand Tertiary rocks. The valves of Trigonia are beautifully pearly within, equal,radiately ribbed, with an external ligament, and a few strong striated divergenthinge-teeth. The umbones are inclined posteriorly—a very unusual feature inbivalves. The foot of the animal is large and powerful, used in crawling amileaping, and without a byssus. In some of the Jurassic rocks of Weymouthtrigonias form a bed several feet in thickness. Mussels (family Mytilidoe) aresuch well-known shells that a description is unnecessary. They are found all over. COMMON MUSSEL [MytlluS nhi/is), CLOSED AND ATTACHED BY THE byssus (nat. size). 406 MOLLUSCS. the world; one very large species from the shores of California sometimes reachingfully 9 inches in length. They anchor themselves by a byssus, but also have the power of moving from place to place, bycasting off the byssus, extending the footin the direction they determine to proceed,and attaching a byssal thread, whichsupports the animal while the foot isagain extended and another thread process is repeated again and again,and thus progress is made. The structureof the animal may be understood byobserving the accompanying illustration,where a represents the edge of themantle; b, the foot; c, the byssus; ,the foot - muscles; /, the mouth; g, thelabial palpi; h, mantle-lobe; and , theinner and outer gill-plates. To the samefamily belong the date-shells (Litho-domus), which, as shown in the accom-panying illustration, are stone - shells are
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
