REPRODUCTION IN THE UNIONID^E 91 
quite tenacious. In a short time after entering the marsupium, 
the eggs become conglutinated into masses which, as the mucila- 
ginous matrix stiffens, are molded into the exact shape of the 
cavity of the water-tube, of which each mass forms a cast. The 
masses are of course separated from each other by the interven- 
ing interlamellar junctions of the gills. 
Since it is a matter of convenience to have a word to apph" to 
these compact masses, in which the eggs or embryos are held 
together, whether they be plate-like, wedge-shaped, cylindrical 
or of some other form, we shall employ the term conglutinate in 
referring to them, as being perhaps the best one available for the 
purpose. They have generally been called ''ovisacs'' by the 
American systematists, but this is of course misleading, as they 
are not sacs in any sense, but merely masses of eggs adhering to 
one another by means of a cement. 
The conglutinates vary greatly in size and shape in conformity 
with the special conditions of the marsupium existing in the dif- 
ferent types. The commonest form is that of a flat oval plate, 
slightly blunter and thicker above and more pointed and thinner 
below. Such plates, differing, however, in size and thickness, are 
characteristic of Lampsilis, Quadrula, Unio, and many other gen- 
era. In fig. 27 two of the conglutinates of Lampsilis ligamen- 
tinus are represented, one from the flat side, the other on edge. 
In those genera in which the size and form of the water-tubes of 
the marsupium depart more widely from the usual condition, the 
conglutinates are similarly modified. In Obliquaria reflexa, for 
example, in which the marsupium consists of several elongated 
and distended water-tubes in the middle region of the outer gill, 
the conglutinates are unusually large, being slightly curved cylin- 
drical masses of nearly uniform diameter and generally blunt at 
each end. Three of them are shown in fig. 26. The one on the 
right was taken from the most posterior water-tube of the marsu- 
pium which is not as long as the rest and its conglutinate is cor- 
respondingly shorter. The relation will be understood by refer- 
ence to the figure of the marsupium in this species (fig. 3). 
In those species, e. g., Unio complanatus, in which the antero- 
posterior diameter of the water-tube is scarcely greater than that 
