In this group the gills are smooth, with the filaments directed downwards, reflected, and connected one with another by interfilamentar ciliated junctions, but the lamellæ are not connected. The foot is usually provided with a well-developed byssal gland. Anomia, Arca, Trigonia, and Mytilus belong to this order.
Cases 137–138.
The family of Anomiidæ contains a number of more or less pearly shells remarkable for a deep notch or hole in the lower or flat valve through which a shelly plug passes, by means of which the animal attaches itself to other shells, stones, &c. Anomia ænigmatica is found adhering to leaves in mangrove-swamps.
Case 138.
The Placunidæ, sometimes called Window-shells and Saddle-Oysters, are very flat pearly shells with a remarkable hinge, which consists of two long divergent teeth, like a ⋀, to which the ligament is attached. The species are few in number, and inhabit sandy shores of India, China, and North Australia.
Cases 139–141.
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.
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.
