XXIV SYKOPSISOF 



the existing numerous species — the exo-skeletons' (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 disj)osed 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, aflibrd 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 Avhich 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. Raf. Naidea. Swain. 



Obovaria. Eaf. Niaa. Swain. 



Hyridella. Swain. Lampsilis. Eaf. 



Canthyria. Swain. Iridea. Swain. 



Eotundaria. Eaf. Quadrula. Eaf. 



Diplodon. Spix. Djsnomia. Agass. 

 Metaptera. Eaf. 



In vol. iii. of the Trans. Am. Phil. Soc, p. 398, Mr. ]S'icklin expressed the oi)inion 

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

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

 g-enus."^ 



In consequence of the enormous extent to which the Family Unionidce 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 genei'a, Margaron^ and Platiris. The first is divided 

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



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

 forces. 



^ 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 Raiinesque. 



^ In the second edition, I projjosed the name of 3Ia7-garita, but, this name being pre-occupied hy Dr. 

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

 Unio). 



