48 THE CHICAGO ACADEMY OF SCIENCES. 
generally large and finely developed. The beautiful develop- 
ment of the shells is due, probably, to the abundance of lime in 
this region. Geologically the family dates back to the Triassic 
Period. 
Pearls are frequently found in our fresh-water bivalves 
(Unio) and are caused by some irritating substance which be- 
comes lodged between the mantle and shell. These pearls are 
similar in structure to the shell, consisting of threelayers. The 
beautiful iridescence is due to light falling on the edges of 
transparent plates. The nucleus of the pearl may be any for- 
eign substance, like a grain of sand, piece of food particle or 
parasitic worm. Spherical pearls are frequently found loose in 
the soft parts of pelecypods, particularly in the muscles. The 
pearls of our fresh-water shells are usually of but little value. 
Parasites commonly infest the Unionide, and Asfpidogaster 
conchicola is found in colonies in the pericardium of Unio and 
Anodonta. Another species, Cotylaspis insignis, is found in the 
upper branchial cavity of Anodonta. Unio may also be consid- 
ered a scavenger, since it feeds on dead animal matter whenever 
it is within reach. 
It is not generally known that in the North American 
Unionide the genital organs may be divided into three groups. 
The first group includes such species as /igamentinus Lam., 
rectus Lam., alatus Say, etc., in which the posterior part of the 
outer branchi@ are used as ‘branchial uteri.” In this group 
the females are readily distinguished by the dilated postero- 
ventral surface, and the uteri are filled with embryones from 
late summer to the beginning of or even through the winter, 
and in the early summer the embryones are discharged. Inthe 
second group all four branchie are used throughout as bran- 
chial uteri, and there is no striking differences between the 
sexes. Examples of this group are pustulosus Lea, @sopus 
Green and wndulatus Barnes. In this group, also, the uteri are 
free from embryones during the fall from about August, and prob- 
ably all winter. During the early summer the branchiz are 
filled with embryones and are discharged some time during 
July. 
Ina third group, which has plain oval to elongated shells, the 
embryos are found in the outer gill alone, filling the whole or 
the greater part of the length of it. In this division may be 
placed the true Unios, the Anodontas and some smaller groups. 
Besides these three great sections there are several smaller ones 
