336 The American Naturalist. [April, 
ON THE VALIDITY OF THE GENUS MARGARITANA. 
By Cuas. T. SIMPSON. 
In 1817, Schumacher founded the genus Margaritana' for the 
Mya margaritifera of Linnaeus, the Unio margaritifera of subse- 
quent authors, on account of the fact that, whereas the shells 
of the latter genus had both cardinal and lateral hinge teeth, 
this species had only the cardinals. Say’s Alasmodonta, applied 
in 1818’ to other Naiads having similar teeth, is synonymous. 
A number of forms have since been added to the groups, mostly 
by Dr. Lea, and, as it stands to-day, it includes some 26 or 27 
species, all confined to North America, with the exception of 
the type, which is circumboreal. 
The soft parts of the different members of this assemblage 
do not differ generically from those of Unio, and any separa- 
tion from that genus can only be founded on the character of 
the teeth which I have mentioned. The Margaritanas do not, 
taken as a whole, form a natural group, but are, undoubtedly, 
polyphyletic in their origin, several of them being evidently 
much more nearly related to certain Unios than they are 
among themselves; and it is only reasonable to suppose, when 
the facts are all carefully considered, that most, if not all the 
species, have sprung from different groups of Unios.’ 
The genus Unio may be divided into a large number of sec- 
tions, which are, I think, unworthy of subgeneric rank, but 
which consist of species that are shown to be closely related by 
characters of the animal and shell, by habits and the facts of 
their distribution. In a majority of these groups, though cer- 
tain species may be considered fairly typical, a close relation- 
ship is shown to other groups by species which seem to stand 
| Essai d'un Noveau Syst. des Habits, des vers Testaces, p. 137, 1817. 
? Journ. Phila. Acad. Nat. Sci. I, p. 459, 1818. 
* The earliest recorded species referred to Margaritana is, I believe, the M. 
nebrascensis Meek (Rep. of the U. S. Geol. Surv. of the Territories, Vol. IX, p- 
114, 1876), from the Upper Missouri Cretaceous. The genus Unio is now believed 
to date back into the Triassic. 
