288 THE CHICAGO ACADEMY OF SCIENCES. 
114. Planorbis trivolvis Say, pl. xxxii, figs. 7, 8, 9, 10. 
Planorbts trivolvis Say, Nich. Ency., pl. ii, fig. 2, 1817. 
Bulla fluviatilis Say, Journ. Phil. Acad., Vol. II, p. 178, 1821 
Planorbis lentus GOULD, Invert. of Mass., p. 202, fig. 1382, 1841. (Not of 
Say.) 
Planorbis regularis LEA, Trans. Amer. Phil. Soc., Vol. IX, p. 6, 1841. 
(Juvenile?) 
Planorbis megastoma DE Kay,N.Y. Moll., p. 61, pl. iv, figs. 60, 61, 1843. — 
Physa planorbula DE Kay, N. Y. Moll., p. 76, pl. v, fig. 83, 1848 (juve- 
nile). 
Planorbis trivolvis var. fallax HALDEMAN, Mon., p. 15, pl. iii, figs. 1-3, 
1844, 
Planorbis macrostomus WHITEAVES, Can. Nat., Vol. VIII, p. 113, fig. 
1853. (Variety.) 
Planorbis plexata INGERSOLL, Survey of the Terr, p, 402, 1874. 
Planorbis distortus W. W. CALKINS, MSS. 
Cochlea trium orbum LISTER, Conch., pl. cxl, fig. 46. 
Shell: Sinistral,* discoidal, flat, carinate above, subcari- 
nate below; color brownish or chestnut horn; surface shining, 
Fic. 96. 
Radula of PLANORBIS TRIVOLVIS Say. (Original.) c, central tooth; 
1, first lateral; 8, intermediate marginal; 14, seventh marginal. 
lines of growth oblique, numerous, raised, equidistant; apex 
almost submerged in the coils of the whorls; whorls four, dis- 
coidal, rounded, subcarinate below, carinate above; spire flat, 
in the young perfectly flat, but in the adult sunk below the level 
of the last whorl, always exhibiting all the volutions; sutures 
rather deeply indented; periphery rounded; base of shell in- 
dented, showing from two and a half to three volutions, which 
*There seems to be no question concerning the sinistral character of the majority of the 
genus Planorbis. The young of all the larger species are sinistral and very much resemble 
some species of Physa. For a detailed study of this subject see STEARNS, Proc. Phil. Acad., 
p. 92, 1881, and BAKER, Jour. Cin. Soc., N. H., Vol. XIX, p. 45, 1896. 
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