The Lucinidæ are almost invariably white shells, and may generally be recognized by the very long muscular scar in front on the inner surface of the valves. They occur in all parts of the world; and the fossil forms, which are still more numerous than those now living, have existed at every epoch from the Silurian.
Cases 166–177.
Of the freshwater Mussels or Unionidæ more than 1200 species have been already discovered; they are found in most parts of the world, the greatest number having been described from North America. In Unio the edges of the mantle are not united along the bottom and not prolonged into siphonal tubes; at the posterior end there are two openings, of which the upper or excretal orifice is simple, and the lower or branchial fringed at the edge. Two “cephalic eyes” have recently been noticed by Mr. P. Pelseneer. The foot is very large and adapted for crawling and burrowing. The sexes are distinct; and the shells of the females are somewhat more tumid than those of the males. Margaritana margaritifera, (Case 168) which is found in this country and in Europe, sometimes produces handsome pearls, but not equal to those obtained from the pearl-oyster of tropical seas. The hinge in this family is extremely variable, being in some instances delicate and toothless (Anodonta, Fig. 31, B), whilst in others it is enormously thickened and furnished with strong interlocking teeth (see Fig. 31, A).
Fig. 31.
Case 178.
The family Ætheriidæ, or freshwater Oysters, consists of but three genera: Ætheria contains African, and Mülleria and Barttettia Indian and South-American forms. When young the shells of Ætheria (which are common in the Nile) are free and not unlike an Anodonta, but when adult they become attached and irregular and look like an olive-green Oyster; they are, however, provided with two muscular impressions instead of one, as in ordinary marine Oysters. Still more remarkable is Mülleria lobata of Colombia, which, when young, freely moves about and has two adductor muscles, but in time becomes attached and stationary, and then possesses but a single adductor.
Cases 179–181.
The Tellinas have usually thin shells, and their two siphons are longer and more completely separated from each other than in the Veneridæ. The pallial line is widely and deeply sinuated, and the ligament generally external. In the genus Semele of the family Scrobiculariidæ, it is placed within the hinge-margin. They live in great numbers beneath the sand in shallow water, and are occasionally used as food.
Cases 183–185.