30 THE NAUTILUS. 
from any Unios; and this seems to be the character on which the 
writer would separate Margaritana generically from Unio. 
In the former species these little musele sears or points of attach- 
ment of the mantle are sometimes a set of round, deep punctures in 
the nacre, but more often they consist of slightly indented dashes, 
which radiate from the umbonal cavity. They vary in number from 
a very few to 50 or more, and are often entirely wanting. In some 
examples these scars are more or less aggregated into a sort of lon- 
gitudinal row along the middle of the disk, looking like a strongly 
developed pallial line. 
In Margaritana monodonta they appear usually as deep punctures, 
and vary from many to none and the same thing is true of Unio hem- 
beli. I have not found them in U. decumbens or U. laosensis. 
In 1830 Isaac Lea described Unio trapezoides in the Transactions 
of the American Philosophical Society, Volume IV, page 69, and 
called attention to the fact that this species possessed a strongly 
developed muscle scar near the center of the disk, which he then 
named the ventral cicatrix. It is present (sometimes double) and 
well developed in most specimens, feeble in others, or it may be found 
in one valve and wanting in the other, or absent altogether. The 
same is true of most of the species of the plicate group of Untos, 
which are all nearly related; N.multiplicatus, undulatus, perplica- 
tus, ete., but I have never found these scars in the nearly allied U. 
sloatianus Lea, of Georgia, which is so close to U. trapezoides that 
Call has placed it in the synonymy of that species.' In U. trape- 
zoides there may be one or two anterior pedal scars and they are 
often widely separated. 
A wonderful degree of variation is also found in the number and 
position of the dorsal scars of many species of Unios, and in the 
degree of development of the scars in the pallial line. In Mr. B. 
H. Wright’s new Unio,—U. bursa pastoris, from Tennesseee, the 
pallial line is generally composed of deep, strongly marked scars, 
to which the mantle is attached ; in Unio ventricosus it is often so- 
faint as to be scarcely discernable. J know of no character more 
variable and wholly unreliable as a means of classification in the 
Unionide than that of the muscle scars and my studies lead me to. 
believe that it is seldom a mark of even specific value. 
1Tr. Acad. Sci. St. Louis, VII, No. 1, p. 54. 
