. Pearls and pearling . material. Another shell which is very similar and is sometimesthought to be the same shell is The Quadrula tubercu-hita, or purple warty back. This variety often growsto be larger than The Quadrula pustulosa, but is not valu-able for buttons on account of its dull purple nacre. Itis not found in large beds like some varieties, but is usu-ally mixed with other shells. This variety is mentionedbecause it is an important shell in the pearl hunting in-dustry. The purple warty-back is the most liable toinfection, by distomids, of all the heavy shelled is a great
. Pearls and pearling . material. Another shell which is very similar and is sometimesthought to be the same shell is The Quadrula tubercu-hita, or purple warty back. This variety often growsto be larger than The Quadrula pustulosa, but is not valu-able for buttons on account of its dull purple nacre. Itis not found in large beds like some varieties, but is usu-ally mixed with other shells. This variety is mentionedbecause it is an important shell in the pearl hunting in-dustry. The purple warty-back is the most liable toinfection, by distomids, of all the heavy shelled is a great producer of slugs and baroques and is verylikely to produce fine pearls. All the slugs, baroques, orpearls found in this variety are purple and dark purple,corresponding to the color of the nacre of the shells. The Tritogonia tuberculata is a very odd shell. It iscommonly called deerhorn, or buck-horn, or pistol-grip. or fan-tail, and is shown in Fig. 16 about one-third natural size. The ones found in the upper portion. Fig. 16. The Tritogonia iubcrciilata, or Buck-horn, or 17. The Quadnda lachrymosa, or Stranger, or 18. The Plagiola securis, or Butter-fly. 82 Pearls and Pearling of the Mississippi River are usually white, with a pearlywhite nacre, and are excellent material for making but-tons and knife handles. They are not always valuable,however, for in some rivers in Tennessee these are pinkand in some Arkansas streams they are purple, and insome of the rivers in Texas they are a deep purple buck-horns do not usually grow in colonies, butare distributed among other shells. Some very valuablepearls are occasionally found in them. The Quadntla lachrymosa, or stranger, or maple-leaf. is illustrated in Fig 17, about one-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 she
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