CONCHIFERA. 321 



and are frequently covered with a homy epidermis, which is 

 thick and olive-coloured in fresh-water tribes, as in the 

 genera Unio and Alasmodon ; pilose or covered with a 

 velvet down, as in Area and Axincea ; drawn out into long 

 beards or filaments, as in Modiola ; developed beyond the 

 edge of the shell, as in Solemya and Cyrtodaria ; or pro- 

 longed over the bases of the siphons, as in Glycimeris and 

 Mya. The apices of the valves constitute the beaks, which 

 are usually straight and pointed, as- in Mytilus, or slightly 

 curved, as in Cardium and Venus, but sometimes they are 

 contorted or spiral, as in Chama and Bucardia. Tbe beak 

 or umbo is always directed towards the anterior side of the 

 shell, which is usually the shortest. The right valve is 

 often smaller than the left, and is regarded by Dr. Gray as 

 the homologue of the operculum in Gasteropodous families. 

 The hind part of the shell may be known by the ligament, 

 which is placed behind the beaks, and by the notch or 

 sinus in the pallial line. The fore part of the shell is 

 occasionally the longest, as in Solemya, and the valves are 

 often nearly equilateral, as in the Pectinida ; sometimes 

 they gape behind for the passage of the siphons, as in 

 Myidie; but occasionally the gape is ventral, as in Ga- 

 leomma, or anterior, as in Rocellaria, in which case the 

 aperture is for the protrusion of the foot. The valves are 

 united at their hinge by a homy, fibrous ligament, which is 

 usually external, and of the same texture as the epidermis; 

 the cartilage is contained in a cavity or "chondrophore" in 

 the hinge-margin, and is elastic and fibrous, keeping the 

 valves habitually open, the closing of the valves .being 

 effected at the will of the animal; in some genera it is 

 contained in a spoon-shaped process, as in Mya, Mactra, 

 and Anatina ; in others it is lodged in several pits along 



