UNIONIDiE. 137 



Sowerby, have gone so far as to propose the union of all 

 the genera in one, whilst others, as Mr. Swainson have 

 maintained the other extreme, and not only divided the 

 tribe into numerous genera, but have even grouped them 

 into sub-families. The tribe, as a group, is a very natural 

 one ; the genera are, we fear, too nearly inclined to arti- 

 ficial sections, and of the extreme opinions, that of Mr. 

 Sowerby approaches nearest the truth. There are, how- 

 ever, in the family a few types of character, round which a 

 number of species seem to group themselves naturally : the 

 sections Unio and Anodon to which, with most authors, 

 we refer our British species, are of this nature and may 

 conveniently be retained. The value of the generic divi- 

 sions at best, however, in this tribe, is not equal to that 

 assumed by such sections in most of the other tribes we 

 have passed in review. 



The Unionida, all inhabit fresh-waters. But few com- 

 paratively are found in the old world, by far the greater 

 number being inhabitants of America. In the United 

 States alone there are more than two hundred species. 

 The collections made by Major Cautley and Dr. Falconer, 

 among the Siwalik fossils would seem to indicate that 

 during tertiary epochs the species of the old world were 

 more numerous than they now are. The tribe ranges far 

 back in time. All the species are very variable, and it is 

 extremely difficult to assign their precise diagnoses. 



The bisexuality of the animals of this family was first, 

 we believe, observed by Professor Bell in Europe, and by 

 Dr. Kirtland in America. The shells of the females are 

 more ventricose than those of the males. 



The distribution of the British fresh-water mussels is 

 peculiar and significant of the four species we possess. Three 

 are members of the genus Unio and one of Anodon. 



VOL. II. T 



