The buckhorn (Tritigonia verrucosa) is very long and narrow; on the dark brown exterior it is rough, as is the horn from which it takes its name, while the interior shows a beautiful display of colors. This is not found in beds, but lies scattered among other species. It sells at a relatively high price—usually in excess of $20 per ton—for button manufacture.
Another species is the butterfly (Plagiola securis), which is very prettily marked on the outside with faintly colored dotted stripes of varying length. Over a background of dark yellow run black stripes to the outer edge of the shell, with dark dots between the stripes. The shell is small and thick, and like the sand shell and the buckhorn, is found in small quantities. Owing to the beauty and permanency of its luster, this shell is in demand for button manufacture, and its pearls are often very beautiful.
Other well-known species are the pancake (Lampsilis alatus), the maple-leaf (Quadrula wardi), and hackle-back (Symphynota complanata). On the Atlantic seaboard, the principal species in which pearls have been found are Unio complanata; the Alasmodon arcuata, which has hinge teeth, and a species of Anodon. Pearls from the Unio complanata are usually smaller but more lustrous than those from either of the other species.
Among the many fresh-water mussels are found some remarkable conditions of animal life. Probably the most curious is the parasitic stage of certain species. When hatched from the egg, each one of these is provided with hooks or spines, by means of which it attaches itself to the gills or fins of a swimming fish and becomes embedded therein. After confinement in this cyst for a period of two months or more, the small mollusk works its way out and falls to the bottom of the river or pond, where its development continues along lines more conventional to molluscan life.
In most of the species of Unios the sexes are separate; but it has been determined that in some the individuals are provided with both sets of sexual organs. It is claimed by some naturalists that certain species may change from one sex to another; yet this does not seem to have been positively established.
Not the least interesting of the habits of the Unios is the manner in which they “walk,” bushels of them changing their habitation in a few hours. The shell opens slightly and the muscular tongue-like “foot” is thrust out, and by pressure of this on the bottom, the mollusk is propelled in a jerky, jumpy movement with more speed than one would suppose possible for the apparently inert creature.
The number of eggs produced by an individual in one season ranges from a few hundred in some species to many millions in others, as in the Quadrula heros, for instance. Most of the fresh-water mollusks are of slow growth, reaching maturity in six or eight years, and it is believed that if undisturbed they live to be from fifteen to fifty years old; indeed, some writers credit them with attaining an age of one hundred years.
While outwardly there is no positive indication of the existence of pearls, they are relatively scarce in young mollusks, and likewise in those having a normal, healthy appearance, with smooth exterior free from blemishes, and they are found generally in the older, irregular, and deformed shells, which bear excrescences and the marks of having parasites. However, some of the choicest pearls have come from shells relatively young and apparently in perfect condition.
It has been pointed out that with the fresh-water Unios there are three indications on which the fishermen to some extent rely for determining the presence of pearls from the outward aspects of the shell. There are, first, the thread or elevated ridge extending from the vertex to the edge; second, the kidney-shape of the shell, and third, the contortion of both valves toward the middle plane of the mollusk.
A single mollusk may contain several small pearls,—more than one hundred have been found,—but in such cases usually none has commercial value. Ordinarily only one is found in the examination of very many shells. Of these objects it may be truthfully said that “many are found, but few are chosen,” few that are of first quality or are worthy of a fine necklace. In many instances, several pounds of cheap pearls would be gladly exchanged for a choice gem weighing an equal number of grains.