The _Arcidæ_ are a family of strong ponderous shells varying much in form and sculpture. The animals have a longish pointed foot, deeply grooved along the bottom, no labial palpi, and free margins to the mantle, which are not prolonged into breathing-siphons. Many of the Arks often anchor themselves by means of a strong byssus. The shells of this family are usually radiately ridged; and the hinge is composed of a number of teeth arranged along the hinge-line, which is generally straight. _Arca tortuosa_, from China, has the valves curiously twisted. The section _Barbatia_ is remarkable for the coarse fibrous character of the epidermis; _Scapharca_ for its unequal valves; and _Cucullæa_, from the Indian Ocean, for the elevated ridge bounding the posterior muscular impression. _Glycymeris_ (better known as _Pectunculus_) has the hinge-teeth arranged in an arched series, and the shells are more regular in growth than in many other forms of _Arcidæ_. [Sidenote: Case 141.] The _Trigoniidæ_ are one of those families which have all but disappeared during our period. Only three or four living species are known, whilst more than a hundred fossil forms have been described from the Jurassic and Cretaceous formations. Australia, where some of the oldest types of animal life persist, furnishes also the existing species of _Trigonia_ (Fig. 27). The animals have a long, sharply-bent, pointed foot like the Cockles, with which they make surprising leaps. The shells are beautifully pearly within, and ribbed and noduled exteriorly. [Illustration: Fig. 27.[8] _Trigonia margaritacea._ Case 141. ] [Sidenote: Cases 142–145.] The _Mytilidæ_, or Mussels, are too well known to need description. The small foot, which is brown in the common species, is not much used in creeping about, but has the power of spinning a byssus or bundle of tough threads, by means of which the animals attach themselves to rocks and one another, forming colonies of vast numbers. Mussels have always been much eaten in this and other maritime countries, and large quantities are brought to the London market from the Dutch coast. At times they are unwholesome; but all the exact causes of this are not known. Mussels seem to be found on every shore, and some of the species are very widely distributed—the common edible Mussel, _M. edulis_, being found on every European coast, on the shores of North and South America, in the Arctic and Antarctic Oceans, and probably on the coasts of Australia. One group of Mussels (_Lithodomus_, Case 144) burrow in rocks and other shells, forming holes just large enough to contain their shells. _L. dactylus_ is sold as an article of diet on the shores of the Mediterranean. PSEUDOLAMELLIBRANCHIA. [Sidenote: Cases 145–160.]