CcUl — The Unionidce of Arkansas. 3 



not complete, we too well know, — will eliminate from the 

 trade-lists of amateurs species names which should no longer 

 burden our faunal lists. In the matter of other bibliographic 

 references the rule adopted has been to give the volume, page, 

 plate, figure and date reference where the form was first 

 described ; added to these are occasional references to well- 

 known and easily accessible works. There has been made 

 no attempt, in the case of the greater number of forms 

 listed, to exhibit but a moiety of the bibliographic matter. 

 Beginning with Uiiio elegans Lea, references have been made 

 to Reeve's great work, the Conchologia Iconica ; this has 

 been done in the hope that those to whom access is given to 

 that work will find the critical notes, which are sometimes 

 appended, of service. Many American shells are therein 

 wrongly named but the fault lies in the sending abroad of 

 misnamed specimens of American Unionidoe. Some of these 

 errors are corrected by Reeve in the addenda to the Genus 

 Unto; the rest remain to puzzle the naturalist. The oppor- 

 tunity to examine and use this great and costly work was 

 afforded by the generous courtesy of Mr. Truman H. Aldrich, 

 of Cincinnati, who kindly loaned me these volumes for a long 

 period of time. 



Not the least interesting fact connected with the study of 

 the Unionidce is the one that numerous species have been 

 duplicated by describing the forms assumed by the sexes as of 

 specific value. This has occurred in a number of instances, 

 and is responsible for a considerable number of synonymic 

 names. Among the forms so described, the following may 

 stand as examples : — 



Unio donaciformis Lea is the female of Unio zigzag^ Lea. 



Unto ater Lea is a synonym of Unio purpuratus Lamarck, 

 and is based on the female form. 



Unio patulus Lea is based on the female of Unio clavus 

 Lamarck. 



Unio lens Lea and Unio leibii are both synonyms of Unio 

 circidus Lea and are based on the female form. Unio leibii 

 is a dwarfed form of U. circulus. 



Unio brevidens Lea is a male form of a species the female 

 of which was afterwards described as Unio arcceformis Lea. 



