6 SYNOPSIS OP 



Of the subgenus Margaritana there are 20 admitted species, and 2 

 whicli are unknown to me. 



Of the subgenus Dlpsas I know of but 2 species, both of which 



are recent. 



Of the subgenus Anodonta there are 5S admitted species, and 7 

 which are unknoMU to me. Of fossil species there is one which is 

 doubtful. 



The subgenus Iridlna has 2 species, both recent. 



The subgenus Spalha has 6 species, all recent. 



Most of the distinguished authors who have written on the subject 

 of the division of the Family Naiades of Lamarck, have acknowledged 

 the extreme dilTicuUy they have encountered in separating it into sub- 

 divisions. This dilViculty is not peculiar to the Naiades. In most of 

 the families where a great number of species have been observed, we 

 find these species so merging, and in some of their characters so fading 

 away into each other, that we scarcely know how, indeed in some 

 instances it is impossible, to make the separation with precision. -Na- 

 tura non facit saltum." In the vegetable kingdom the same obstruc- 

 tions to a svstem are encountered. The observations of Lindlev* are 

 so just and philosophic, that I cannot refrain from quoting them here :~ 



" Species are created by Nature herself, and remain always the same, 

 in whatever manner they may be combined : they form the basis of 

 all classification, and are the only part of it which can be considered 

 absolute. For although in a natural system, all other combinations, 

 whether genera, tribes, orders, or by whatever name they may be 

 know^n, comprehend species agreeing much more with each other than 

 with any thing else, and having a positive general resemblance in the 

 majority of their features, yet no fixed limits can be assigned to any of 



* See Introd. to Botany, p. 307. 



