A CLASSIFICATION OF THE AMERICAN OPERCULATE 

 LAND MOLLUSKS OF THE FAMILY ANNULARIIDAE. 



By John B. Henderson and Paul Baetsch, 



Of the Section of Mollusls, United States National Museum. 



INTRODUCTION. 



This new classification of the American "Cyclostomidae'' is the 

 outcome of the critical study of an almost complete collection of the 

 known species comprising thatlargeand diversified assemblage of oper- 

 culate land-snails. Opercular characters have been employed for aU 

 the larger groupings, that is, the subfamilies and genera. The com- 

 paratively few species, the opercula of which we have not actually 

 seen, are excluded from consideration. We soon found that assump- 

 tions as to generic position of a species based upon similarity of shell 

 characters were too often misleading. To the total of the species 

 involved the proportion of those of which our specimens lacked oper- 

 cula, while not negligible, is yet not sufficiently great materially to 

 weaken our conclusions, nor likely, when their opercula may even- 

 tually be known, to call for any serious modification of our scheme of 

 classification. 



The new arrangement here offered calls, first, for a separation of all 

 the American forms from the Old World groups with which they have 

 been associated. This is accomplished by the creation of a new 

 family, the Annulariidae, founded upon a constant and essential 

 radular difference, the details of which are discussed under the descrip- 

 tion of the new family. 



The second step has been the creation under the Annulariidae of 

 four chief groups designated as subfamilies. These subfamilies are 

 based wholly upon characters presented by tlie operculum — charac- 

 ters which have been accepted as basic and of primary importance. 

 The natural order or sequence of these subfamilies has been deter- 

 mined by what we conceive to be progressive opercular changes 

 through various gradations from the simplest or most primitive form 

 to the most comphcated or specialized types. These changes appear 

 to proceed by easy gradations and with but slight interruption through 

 continuous lines of development. 



The third step has called for the |)roper disposition of the existing 

 genera and for the creation of a few new genera. These are based 



Proceedings U. S. National Museum. Vol. 58— No. 2327. 



181404— 21— Proc.N.^Sr. vol. 58 4 4!) 



