154 AMERICAN JOURNAL 



Skenein^, witli a depressed, almost discoidal shell, and a 

 corneous, paucispiral* operculum. Minute. Marine. Genus 

 Skenea, Flem. 



Hydrobiin^, with shell and operculum and foot like those 

 of the Bissoinse, but with the Rhachidian tooth of the lingual 

 ribbon having the basal teeth on the anterior surface behind 

 the lateral margins. Size variable ; some are minute, some as 

 large as Bytlmiise. Living in fresh or brackish water. Genera 

 irydrohia,\ Hartm., Littorinella, Braun, Amnicola, Gould and 

 Hald., Bythinella,% Moq.-Tand., Stenothyra,\ Benson, Trincula, 

 Benson, Pyrgula, Christ, and Jan., Paludestrina, D'Orb., Try- 

 onia, Stm., Potamopyrgus, Stm., Lithoglyplms, Muhlfeldt, Flu- 

 minicola, Stm., Gillia, Stm., Somatogyrus, Gill, Cochlioj^a, Stm. 



POMATIOPSIN"^, with the shell and operculum as in the Bis- 

 soinae. Foot with lateral sinuses. Size small. Amphibious. 

 Genus Pomatiopsis,^ Trjon. 



The above mentioned characters may be tabulated as fol- 

 lows : — 



A. Operculum concentiic - - - -" - - Bythiniin^. 



B. Operculum spiral. 



a. Operculum pauci-spiral Skenein^. 



b. Operculum subspiral. 



1. Operculum with an internal process - - Rissoininje. 



2. Operculum without an internal process. 



* Foot without lateral sinuses. 



t Rhachidian tooth of the lingual rib- 

 bon with the basal denticles on 

 the inferior margin - - - Rissoin^. ] 

 I Rhachidian tooth of the lingual rib- 

 bon with the basal denticles on 

 the anterior surface behind the 

 lateral margins . . - . HYDROBiiNiE. 

 ** Foot with lateral sinuses - - - Pomatiopsin^." 



"We beg to differ in opinion from Dr. Stimpson regarding 

 the rank to which these various groups are entitled. In the 

 first place, we think our Amnicolidse as validly separated as 

 a distinct family as are many other well-recognized families 

 of shells. We would mention the following distinctive cha- 

 racters in addition to the only one which Dr. Stimpson 

 finds to belong to the Amnicolidas, that of the "foot not con- 

 tinued in front of the rostrum." 



* According to the terminology of Woodward, the operculum of Skenea 

 would be multi-spiral, and that of the Bissoinm, etc., pauci-spiral. 



t Syn. Paludinella, Loven (not Pfeiffer), and Littorinida^ Eyd. & Soul. 

 § Syn. Leacliia^ Risso (not LesueurJ, Microna, Ziegler. 

 I Syn. Nematura, Benson. 

 *1[ Syn. ChilocyduSf Gill. 



