16 The Molluscan Family Plauorbidae 



The sculpture of the planorbid shell consists mostly of growth lines 

 which may be very fine and thread-like or coarse and rib-like. In some 

 species there have been developed costae or ribs more or less evenly dis- 

 tributed. In a few species the epidermis forms paper-like crests or frills, as 

 in Helisoma anceps royalense. Spiral impressed lines are found in most of 

 the species of the family. These are usually rather heavy and conspicuous 

 but in some species they are very fine. In a few species the spiral lines are 

 very faint or absent, as in Menetus opercularis. Malleated individuals are 

 comparatively rare in the Planorbidae although they are common in the 

 Lymnaeidae. The epidermis or periostracum of the planorbids is usually 

 rather thick and heavy. In some groups the shell appears devoid of an 

 epidermis, the surface being shining and waxy. The remarks on sculpture 

 in the Lymnaeidae Monograph (pp. 3, 4) apply equally well to the 

 Planorbidae. 



The color of the shells of the family Planorbidae is usually some 

 variety of horn color, light or dark. In some groups, as Australorbis, the 

 color is rich brown or chestnut. Some shells are light milky in color. A few 

 species border on red, others are ashy. The range of color is about like that 

 in the family Lynmaeidae, but there are no species known comparable to 

 the greenish shell and purplish aperture of Bulimnea magasoma, the hand- 

 somest of the fresh w^ater pulmonates. Zebra markings occur among the 

 planorbids as among the lymnaeids. 



