FAjMILY LYMN.tlUyE 85 



would (lifferciuiatc Tapliiits Adams from Ilelisoma ; and Anisopsis 

 Sandbcrger, from the Jura, is very similar, though tlic aiicrlurc is not 

 preserved in the fossils and may have been simple. 



Section Pierosoma Uall Shell large, higli, with few transversely 

 sculptured whorls ; the early whorls carinate and flattened above, 

 funicular below ; in the adult the flattened apex is usually depressed 

 below the upper level of the ultimate whorl ; the aperture is suddenly 

 expanded and thickened. Type P. trivolvis Say. 

 Section Planorbella Haldeman. Like Uclisotiia but smaller, with more 

 numerous whorls, with the last whorl strongly constricted beiiind a 

 campanulate aperture ; a flattish or even slightly convex upper sur- 

 face ; the base funicular. Type P. campanulafus Say. 

 The P. vmltivolvis Case differs from the type of P/afiorbella chiefly 

 by its more numerous and closely coiled early whorls. In both a 

 second year's growth shows a varix due to the retention of the aper- 

 ture of the preceding year. Two preoccupied names were applied to 

 P. umltivolvis by Adams, but a study of specimens leads to the con- 

 clusion that its separation is unwarranted. 



Subgenus Tropidiscus Stein. Shell depressed, the adult periphery 

 angular or carinate, the aperture oblique, slightly expanded, simple. 

 Type P. wubilicatus Miillcr ( -|- /*. complanattis Stein). 

 Section Tropidiscus s. s. Shell moderately large and with compara- 

 tively few rapidly increasing whorls of which the junior portions 

 are not keeled. Type P. uvibilicatus Miiller. 



This subgenus was called ' Gyrorbis Agassiz,' by Moquin Tan- 

 don, but Agassiz never proposed any such genus or group, the name 

 Gyrorbis having been applied to a subdivision of Valvaia by Fitz- 

 inger. Moquin Tandon's error was copied by Gray, and later by 

 Westerlund, who, still later, having become aware that Fitzinger's 

 name existed, proposed for the group already named by Stein, the 

 name Diplodiscus ; which naturally becomes a synonym of Tropi- 

 discus Stein. Nevertheless, since Westerlund arranged his really 

 typical ' Gyrorbis ' under Tropidiscus, and grouped under his new 

 name the species of which P. vortex is an example (though without 

 mentioning any type) and gave a suitable diagnosis, it may not be 

 stretching the rules of nomenclature too far to retain his name for the 

 following section. 



Section Diplodiscus Westerlund (restricted). Shell small, with 

 numerous slowly enlarging whorls keeled or angulate from the 

 beginning. Type P. vortex Linn^. 



