DES MOINES ACADEMY OF SCIENCE. 



as herein, the type does not always plainly suggest the affinities of certain forms 

 grouped with it as subordinate members. Thus the pustulate group, typified by 

 Unin pustulosus, contains species which, like Unio Uoitstuneiwls. are non-pus- 

 tulate. But, since there is an essential agreement in all other characters, and 

 since individuals of usually pustulate forms are sometimes fcund free of pus- 

 tules, no real violence is done any scientific principle of classification. Indeed, 

 the presence or absence of pustules often appears to afford a no more reliable 

 basis of classification or grouping than does color, which, as has long been recog- 

 nized, is by no means a constant factor. 



Collections from every sub-drainage area within the Mississippi valley are 

 generally important in order to understand the geographic range of characteristic 

 forms and as an aid in the determination of suspected synonomy, and particu- 

 larly important in connection with the chronologic history of the family in these 

 waters. The existence of various species of TJnlonidw in the Laramie beds of 

 Wyoming, probably genetically related to certain widely spread types in the 

 Mississippi basin, as shown by White (2; point to regions adjacent to the great 

 valley, as the locality of probable origin. When investigation of this phase of 

 the subject is carried further, so that actual genetic relation be demonstrated, an 

 important fundamental problem, that underlies the fact of synonomy, will be 

 solved. 



In brief, then, the intention of this tentative catalogue is to place within the 

 reach of students, a convenient geographic resume of the Unionidce of the Mis- 

 sissippi valley, and at the same time to prepare the way for an intelligent discus- 

 sion of the biologic and geologic problems which their numbers and distribution 

 suggest. The greater fullness of the notes which pertain to certain groups, is to 

 be considered indicative of the opportunities the writer's private cabinet presents 

 and the extent to which he -has been aided by the generous help of a numerous 

 list of correspondents, while the paucity of notes in certain other instances will 

 serve to indicate those species either good or nominal concerning which infor- 

 mation is most earnestly desired. A failure to point out the possible affinities of any 

 form, is sometimes the result of positive ignorance of its true relations consequent 

 upon inability to examine authentic specimens. '1 he element of uncertainty which 

 attaches to all mere descriptions and to most figures, is too great to warrant a 

 suggestion of possible relationship. It could, at best, be no more than mere 

 conjecture. Those suggestions which involve a probable synonomy are advanced 

 with some degree of trepidation due both to the inherent difficulties of the sub- 

 ject and to its delicacy. But the desire, which every honest investigator has, to- 

 render strict justice to the work of his predecessors needs also to be judiciously 



(2) Am. Jour, of Sci., Vol. XX, Pp. 44-49. 



