302 ANNALS OF THE CARNEGIE MUSEUM. 
marsupium toward the posterior part of the gills and the shell, in 
order to have it as close as possible to the branchial opening and the 
inflowing water. 
Of the four types of marsupium, three are found only in compar- 
atively a few forms, while the fourth is more widely distributed, and 
gives origin to a new line of development. The first three may be 
called rather indifferent attempts on the part of the forms concerned, 
to solve the problem. The problem has been solved by them, indeed, 
but the way in which they did it did not contain any further pos- 
sibilities. In the fourth case, the attempt was more successful, and 
opened the way for a series of additional improvements. 
1. In one case (Ptychobranchus), the marsupium remains in a 
primitive stage in this respect, that it is pushed only slightly beyond 
the edge of the gill, and is not moved backward, but occupies the 
whole gill, But here in order to insure proper aération by increasing 
the surface of the marsupium, while the latter remains rather thin, 
the whole marsupium is thrown into a number of folds which permit 
the water to easily reach the ovisacs, which are subcylindrical and not 
much swollen. 
2. In the second type (Obliquaria and Cyprogenia), the task has been 
accomplished by reducing the number of ovisacs. This would have 
had the result of restricting the number of ova that could be accom- 
modated in the marsupium, but this disadvantage is counterbalanced 
by a tendency to greatly elongate the ovisacs, in the direction beyond 
the edge of the gill. This feature is only slightly developed in Obli- 
quaria, while it reaches its greatest perfection in Cyprogenia, and here 
there is not room enough within the shell for the extremely elongated 
ovisacs and thus they have to coil up in a spiral. 
3. In the third type (Dromus), the marsupium originally is rather 
simple, the ovisacs remaining subcylindrical or being only slightly 
compressed. Here a better aération is accomplished by a peculiar 
arrangement of the glochidia within each ovisac. They are not dis- 
tributed through the mass of the placenta, but are situated along the 
edge of the slightly compressed placenta, thus facing the outer walls 
of the marsupium, where they are nearer the breathing water. In 
addition Dromus has developed a peculiar warping and folding of the 
marsupium, which also apparently has the object of increasing the 
surface offered to the water. But this latter feature is exhibited only 
in old specimens. it reminds somewhat of the structure of Ptycho- 
