THE GASTROPOD GENUS YVANIA 

 CONTRIBUTION TO THE PALEONTOLOGY OF ILLINOIS 



By 

 J. Marvin Weller 



INTRODUCTION 



Modern tendencies in paleontology are toward greater discrimination 

 and liner distinctions between genera as well as between species. It is only 

 in this way that paleontology may keep pace with our more exacting strati- 

 graphic studies. This paper on Yvania is the first of a series that has been 

 planned in winch a number of groups of American Carboniferous i'leuro- 

 tomariidae will be described. It is anticipated that in most cases it will be 

 possible to classify these forms under already proposed generic appellations, 

 but it will undoubtedly be necessary to redescribe many of the genera in order 

 that their essential characters may be understood and that a grouping more 

 natural and satisfactory than has been accomplished in the past may be made. 



THE FAMILY PLEUROTOMARIIDAE 



The family Pleurotomariidae includes a very large number of species 

 occurring in strata which range in age from Cambrian to Recent. The family 

 is characterized by a spiral shell-habit and a definitely limited narrow band 

 which terminates in a more or less deep sinus of the outer lip or in a long 

 parallel-sided slit which in some species is closed at intervals. 



Within the family the species differ in almost every conceivable detail, 

 so that it has proved a stumbling block to many paleontologists who have 

 attempted to classify its members. The general practice has been to separate 

 a small group as a new genus or subgenus, but differences between the type 

 s|<ecie> chosen and other representatives of the family have not always been 

 clearly understood, and the result has been an unnatural grouping of species. 

 Conservative authors have referred their species either to Pleurotomaria or 

 tfurchisonia according to a single arbitrary character — length of spire. 

 Others have compromised by classifying in genera already proposed those 

 sjiecies which seem to possess correponding characters and referring the 

 remainder to Pleurotomaria and Mu.rchison.ia. Consequently a large number 

 if species whos< relationships are not sufficiently understood to warrant more 

 precise generic classification are now included in Pleurotomaria. In this 



