ico land and fresii-v\'ater shells or n. a. [part ii. 



Fossil Species of Planorbis. 



Dr. Meek furnishes me with the following list of fossil species : — 

 Planorbis spectabiUs, Meek, Proc. Phila. Ac. 1860, 315. 

 Planorbis utakensis, Meek, Proc. Phila. Ac. 1860, 314. 

 Planorbis vitrinus, Meek & Hayden, Proc. Phila. Ac. 1860, 413. 

 Planorbis nebrascensis, EvAxs & Shdmakd, Proc. Phila. Ac. 1854, 154. 

 Planorbis vetulus, Meek & Hayden, Proc. Phila. Ac. 1860, 175. 

 Planorbis convolutus, Meek & Hayden, Proc. Phila. Ac. 1856, 120. 

 Planorbis planoconvex, Meek & Hayden, Proc. Phila. Ac. 1860, 452. 

 (Olim fratjilis, Meek & Hayden, Proc. Phila. Ac. 1S57, 136, not of 

 Dunkek.) 

 Planorbis subuvibilicatus, Meek & Hayden ^ Valvata subumbilicata, q. v. 



SEGMEXTIXA, Fleming. 



Tentacles filiform. Foot narrow anteriorly, larger behind. 



Shell dextral, discoidal, spire depressed, horn-colored ; whirls 

 few, visible on both sides, furnished internally with transverse, 

 testaceous partitions or teeth ; aperture transversely oval or 

 circular ; outer lip simple. 



Jaws (of S. lacustris) very narrow, very much arched, flexible, 

 scarcely brown, greatly att^iuated, pointed. Vertical stria; or 

 marginal denticulations hardly apparent. 



Lingual membrane — ? 



There are but few species of Segmentina, which are not 

 acknowledged as a separate genus by all authors. The name 

 either as generic or subgeneric is universally adopted, as it has 

 priority of Hemithalamus, Leach, Segmentaria, Swains., and 

 Discus, Hald. 



The typical forms are not represented in this country — our two 

 species belonging to the section Planorhula. 



Subgenus PLANORBULA, Hald. 



Shell with tlie aperture furnished with dentiform plicffi, not 

 forming ^open partitions. 



Segmentina -^^lieatleyi, Lea.— Shell small, dark horn-colored, 

 flat, obsoletely striated, bicariuate, depressed above, broadly and deeply 



