THE NAUTILUS. 19 
Sometimes mature, or apparently mature glochidia and quite un- 
developed ova are found mixed up in the branchial sacks. Whether 
the latter wil] develop into embryones later, or remained unchanged 
for want of impregnation, remains to be studied up. 
2. Branchial sacks, or uteri. On Lampsilis, the branchial sacks 
are differentiated even when not charged with ovaor young. They 
are situated in the posterior part of the outer branchia, in a group, 
the marsupium, which, when charged, is very considerably enlarged, 
often exceeding half the length of the shell, and crowding away the 
unchanged anterior and posterior parts of the branchie. It has 
already been said that the number of sacks is, to a certain degree, 
characteristic for each species, yet rather variable even in individ- 
uals of the same size, and it is also hardly ever the same on the two 
sides. In the young, there are only a few, and their number is 
increasing with the age of the animal. They are also not all of the 
same size, and each one may occupy a smaller or greater number of 
branchial filaments. 
In younger animals, there are always a number of small, empty 
sacks adjacent to the gravid ones, preformed to be charged in the 
following year. 
The shape of the uterus sacks in U. irroratus Lea is known from 
the autbor’s description and figure. There is considerable variation 
in their numbers. Of three specimens from the same place, all 
medium sized, one had seven sacks on one side, four on the other, 
the second had eleven and ten, the third, ten and eight. At the 
proximal ends there were exclusively ova; at some distance, those 
in the periphery had transformed into glochidia, and at the distal 
ends the latter were in excess, while a great number of ova had still 
remained unchanged. In accordance with this, the flesh color was 
much more intense at the proximal than at the distal ends, as the 
ova are colored, the young colorless. The ova are packed closely 
together and coherent by some intermediate substance, so that the 
whole worm-like cylinder can be extracted in toto from the enclos- 
ing membrane. 
The young, in the uterus, show marked differences from those of 
all other species seen, as to soft parts and shell. The latter is con- 
siderably longer than high and has numerous distinct, crowded, con- 
centric lines of growth. Its length is 0-21, alt. 0-17, diam. 0°14 mill- 
imeters. 
* In one specimen, the ova, and so the whole cylinders, were colorless, a rare 
exception. 
