xxiv synopsis or 



the existing numerous species — the exo-skeletons 1 (so to call them) of which have 

 only come under our notice — the facilities which a good system ought to afford can- 

 not be reaped by a partial anatomical knowledge, which does not now embrace, 

 probably, one-fifth of the ascertained species of the family. Besides this, I am not 

 at all disposed to think that we can entirely dispense with the aid we find in the 

 various characters of the exo-skeletons, in making our sub-groups. They often, in 

 fact, afford striking and obvious differences, which the eye schooled with but little 

 experience can with facility and certainty detect, and which, if happily grouped by 

 an experienced eye, may greatly aid the student. These considerations have induced 

 me still to retain nearly the same divisions in this which I used in the third edition, 

 with a few additional divisions. 



Messrs. H. and A. Adams, in The Genera of Recent Molluscs, vol. ii. 1858, adopt 

 the following Subgenera of Unio: — 



Banosta. Eaf. Naidea. Swain. 



Obovaria. Eaf. Niaa. Swain. 



Hyridella. Swain. Lampsilis. Eaf. 



Canthyria. Swain. Iridea. Swain. 



Eotundaria. Eaf. Quadrula. Eaf. 



Diplodon. Spix. Dysnomia. Agass. 

 Metaptera. Eaf. 



In vol. iii. of the Trans. Am. Phil. Soc, p. 398, Mr. Nieklin expressed the opinion 

 "that the seven genera, now referred to the Family of Uhionidce, are founded in arti- 

 ficial distinctions, and not in nature ; and that, in fact, the family contains but one 

 genus." 2 



In consequence of the enormous extent to which the Family Unionidw is expanded, 

 it is absolutely necessary to establish many groups and sub-groups, so as to make a 

 systematic arrangement accessible to the student of this branch of zoology. I there- 

 fore divide the family into two genera, Margaron* and Plaiiris. The first is divided 

 into ten subgenera, and the latter into three subgenera. These are again divided 



1 Forming the calcareous coverings and fulcra for muscular fibres, as well as protection from exterior 

 forces. 



3 The entire uncertainty of the divisions of Mr. Rafinesque, or their multiplied, useless, and incom- 

 prehensible groups, are not deemed important to insert here. Nor the synoptical tables of Mr. Conrad, 

 which differ so much from each other, the last being founded very much on that of Rafinesque. 



3 In the second edition, I proposed the name of 3Iargarita, but, this name being pre-occupied \>y Dr. 

 Leach for a genus separated from Turbo, I substituted in the third edition that of Margaron (^apyapoc, 

 Unio). 



