NATURAL SCIENCES OF PHILADELPHIA. 33 



Systematic Arrangement of the Mollusks of the Family VIVIPARIDJE, and 

 others, inhabiting the United States. 



BY THEODORE GILL. 



My attention having been recently again attracted to the classification of 

 the family of Viviparidae by the printing of the " Descriptive Catalogue" of my 

 friend, Mr. Binney, I propose to offer the following revision of the arrangement 

 of the species of North America. There are four distiact group3 of species op. 

 the continent which some will call genera, and others subgenera. These have been 

 formerly comprehended under the generic name of Paludina, or Viviparus ; the 

 four were first recognized as distinct by Mr. G. W. Tryon, Jr., in " Notes on 

 American Fresh-Water Shells."* The characters which appear to be the prin- 

 cipal distinctive ones have, however, remained unnoticed by Mr. Tryon and all 

 his predecessors, while those assigned to the respective groups have been rather 

 vague and uncertain. A species closely related to, if not identical with, the 

 type of Viviparus V. georgianus ex Lea has been referred by the Adams and 

 CQenu to the subgenus Mclantho, while, on the other hand, a trae 3felantho M. 

 cy do stoma tiformis is placed in Viviparus; the distinctive characters of the two 

 groups, as given by those gentlemen, are by no means obvious. 



It has recently been customary to consider the structure of the operculum 

 as having a paramount value and indicating family distinction. On such 

 grounds, the genus Amnicola has been separated from the family to which 

 Bythinia was referred, and has by some been considered as the type of 

 an independent one ;f while others have referred it to the Melaniidse, % Rissoidse, 

 or Littorinidae,|| Moquin-Tandon, to whom we are indebted for the most perfect 

 account extant of the extra-marine mollusks of any country, has considered those 

 differences to which others have assigned a family value as only subgeneric. That 

 learned malacologist, in his precious work on the land and fresh-water mollusks of 

 France, has combined in the genus Bythinia, the A'mnicolse and BytMnix of 

 American and most other naturalists, giving to the former the sectional name 

 of Bythinella, and to the latter, that of Elona. There can be little or no doubt 

 that those groups differ generically, but they should apparently be approxima- 

 ted to form a family distinct from the true Viviparidx. 



I would then distinguish the two families as follows, my knowledge of the 

 anatomical characters being chiefly due to M. Moquin-Tandon. But, in 

 the first place, the following analytical synopsis of the families of Pectini- 

 branchiates, represented in the fresh- water streams of North America, is offered . 

 Ii this table, only the most striking and not always the most important char- 

 acters are given. 



I. Teeth of lingual membrane 3 | 1 | 3 T/Enioglossa. 



A. Gills concealed. 



1. Rostrum moderately produced and entire or simply 



notched. 



a. Foot not produced beyond head: branchia uniserial. 



^Lateral jaws present. (Aperture of shell acuminate 



behind; generally chanelled at front; size 



moderate) Melaniid.e. 



**Lateral jaws obsolete. (Aperture of shell sub- 

 circular, broadly rounded at front ; size very 



small) Amnicolid.e. 



/?. Foot produced considerably beyond the head ; 



branchifetriserial Viviparidx. 



* Proc. Acad. Nat. Sciences, Philadelphia, 1862. p. 451. f Tryon. op. cit., supra, 1S62, p. 451 



t Lea, Proc. Acad. Nat. Sciences, of Philadelphia. 1862. 



' H. & A. Adams, Chenu, Binney. || Gray. 



1863.] 



