1895.] On the Validity of the Genus Margaritana. 337 
between and partake of the characters of two or more assem- 
blages ; sometimes to that degree that they might, with equal 
propriety, be placed in either of two or three sections. The 
same thing is true of the Margaritanas to some extent, and, 
while a few of the divisions of this so-called genus do not seem 
to be very closely related to anything else, others show such 
strong affinities to certain groups of Unios, that they will have 
to be placed with them in anything like anatural arrange- 
ment. 
In the Mississippi Malley and on the Atlantic Slope, there is 
found a small group‘ of Unios fairly typified by U. pressus, 
which consists of compressed, rather quadrate or rhomboidal 
shells, with strongly undulate beaks, and faintly rayed green 
epidermis. In the right valve of all these forms, the hinge 
plate is cut away directly under the beak, and there is a long, 
curved lateral, and a tolerably perfect, compressed cardinal, 
the latter separated from the edge of the shell by a deep, 
parallel sided socket. In the left valve is a somewhat triang- 
ular, recurved cardinal, which exactly fits into and fills up 
this missing area in the hinge plate of the opposite valve; 
another cardinal fills the parallel sided socket, and there are a 
couple of usually somewhat blurred laterals. In this group I 
should place Unio pressus, tappanianus, charlottensis, neglectus, 
and possibly one or two others. , 
Now Margaritana rugosa bears so strong an external resem- 
blance to Unio pressus, that one is often taken for the other by 
persons familiar with the species; the principal difference 
being that the former is generally somewhat corrugated on the 
posterior slope, while the latter is without this sculpture, 
though this distinction does not always hold perfectly good. 
The arrangement of the teeth is precisely the same in both, 
but in the Margaritana the hinge plate is a little heavier, and 
the laterals are more blurred or imperfect. Dr. Lea carefully 
examined specimens of the animals of both, and it will be seen 
that they are very much alike by the descriptions which I give 
in his own language. Recently, Dr. V. Sterki, a careful anato- 
mist of New Philadelphia, Ohio, who has dissected these spe- 
cies, has reached the conclusion that they are very nearly re- 
lated. 
