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 aeration 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 (Dromns), the marsupium originally is rather 

 simple, the ovisacs remaining subcylindrical or being only slightly 

 compressed. Here a better aeration 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- 



