. Pearls and pearling . third naturalsize. This is very scarce in some rivers, and is not usual-ly abundant in any of the rivers. It is similar to the•three-ridge. It is used in the manufacture of buttons,but the irregular surface of this shell slightly reduces itsvalue as button material. These also produce nice pearls. The Plagiola securis, or butterfly, is a very prettyshell, with pearly white nacre. This shell is shown inFig. 18, one-third natural size. A pair of these shellsopen, and when viewed from the outside, have the ap-pearance of a butterfly, and for this reason, their com-mon name
. Pearls and pearling . third naturalsize. This is very scarce in some rivers, and is not usual-ly abundant in any of the rivers. It is similar to the•three-ridge. It is used in the manufacture of buttons,but the irregular surface of this shell slightly reduces itsvalue as button material. These also produce nice pearls. The Plagiola securis, or butterfly, is a very prettyshell, with pearly white nacre. This shell is shown inFig. 18, one-third natural size. A pair of these shellsopen, and when viewed from the outside, have the ap-pearance of a butterfly, and for this reason, their com-mon name is used very appropriately. This variety is one of the most valuable ones for mak-ing buttons. In some rivers the butterfly attains alarge size. It is not abundant in most rivers, althoughthere are some places where it is quite common. Somevery lustrous and valuable pearls are occasionally foundin this variety. The Lampsilis ventricosus, or pneketbook, i> shownin Fig. 19. about one-third natural size. This variety. Fig. 19. The Lampsilis ventricosus, or 20. The Sytnphonota complanata, or Razor-back, or Hatchet-back. 84 Pearls and Pearling grows sparingly in both lakes and rivers, and is knownthroughout most of the States. In some instances theshell is too thin for button material, but usually it is agood shell for the purpose. The nacre is generally whiteand lustrous, although some times a few pink ones arefound. They are some times used in the manufacture ofpocketbooks and novelties. Slugs, baroques and pearlsare occasionally found in these shells. The Symphonota complanata, or razor-back, orhatchet-back, or heel-splitter, or hackle-back, isshown in Fig. 20, about one-third natural size. This va-riety is common in most rivers, but is not a valuable shell,unless it is thick and has a white nacre. As a rule, how-ever, the shells are thin and colored. They do not growin beds by themselves, but are usually scattered amongother varieties of shells. Pearls are
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