BULLETIN OF THE 



distribution of the combined forms as a unit. In this respect, the treatment of 

 species has not been uniform. Where identities have appeared to be fairly well 

 established, comparisons have been instituted and the results positively stated, 

 though in the great majority of cases there has been no attempt to critically fix 

 the species synonomy. The evidence which is not inconsiderable by which 

 such treatment is justified will, it is hoped, appear in a future synonymic cata- 

 logue designed to cover the same area with this. It is proper to state, in this 

 connection, that the extremes of distribution and such synonomy as is positively 

 asserted, are based upon the private collections of the writer, except in those in- 

 stances where shells are cited as occurring in British America. The statements 

 of distribution in these few instances, are based upon material to be seen in the 

 collections of the Smithsonian Institution. 



In anticipation of the results' of a further study of synonomy it is suggested 

 that the elimination of a large number of forms, herein included as species, is 

 probable. Many others, when studied in connection with the peculiar environ- 

 ments imposed by wide geographic range, will be found devoid of even a racial 

 value, while still others will rank as mere varieties. In the case of several 

 widely distributed groups it has been found that the extremes of differentiation 

 are co-ordinate with the extremes of distribution, and that the differences become 

 less appreciable as the various intervening localities furnish their quota to the 

 geographic series. Such, notably, is the fact in connection with the pustulate 

 group of Uniones in which the northern Unto pustulosus graduates insensibly 

 into the southern Unio asperatus, U. vallatus and U. splicericus. Bearing this 

 fact in mind the co-ordination of species on the basis of resemblances rather than 

 their specific separation on trivial variant characters, appears to be most useful as 

 a preparation for a complete synonomy. Thus, in this paper, well known and 

 common forms are taken as the types of groups in which are placed all those 

 species which present close resemblances, and which are conceived to be if good 

 species genetically related to the type. In general, the first described species 

 of a natural group has been selected as the type, so that on the completion of 

 the synonomy of any individual group the arrangement would be chronologic. 



It has not appeared desirable to institute subgenera for the reception of such 

 forms as approach closely to the type of any group. Several such attempts have 

 been made, (1) but the limits imposed by these devices appear to be no more 

 constant than the characters of the specimens themselves. It is believed that, for 

 the North American Unionidce at least, no more characteristic grouping than 

 that implied in the adoption of the genera Anodonta, Margaritana and Unio 

 \vill be found really serviceable. Even in the adoption of certain typical groups, 



(1) By Raflnesque, L. Agassiz, Swainson, Stimpson, Chenu, H. and A. Adams, and 

 others. 



