ORTMANN: FAMILIES AND GENERA OF NAJADES. 303 
branchus, but I think it has been independently acquired, and does 
not indicate close relationship. 
4. In the fourth type (all other genera), an entirely different ar- 
rangement to provide aération for the marsupium has been effected. 
While in the first three types structural modifications of or within the 
marsupium are introduced to bring the glochidia close to the breathing 
water, here the marsupium itself remains rather simple, and it is the 
water supply which is increased and intensified. 
The marsupium forms in this case a rather swollen, generally kidney- 
shaped mass, in which the ovisacs are transversely dilated, so as to 
give them a leaf-like shape. The tendency to locate the marsupium 
in the posterior part of the gill and to cause it thus to approach the 
posterior end of the shell, close to the branchial opening and close to 
the incoming water, is common to all these genera. In other respects, 
there is no further differentiation of the marsupium. Buta new device 
begins to develop, having for its aim the increase of the flow of the 
water over the marsupium, and this is accomplished by special struc- 
tures on the edge of the mantle, just in front of the branchial opening. 
In the simplest cases (Obovaria, Nephronajas, Amygdalonajas, Pla- 
giola, Paraptera, Proptera), the inner edge of the mantle is only slightly 
dilated, forming a fine lamella, but the presence of a (muscular) 
thickening, and often of pigment, indicates, that the edge has here a 
peculiar function. In other genera (Medionidus, Eurynia, Lamp- 
silis), this part of the inner edge of the mantle is greatly developed, and 
carries special appendages in the shape of strong papille or flaps, 
which have the function of producing by their contractions, a lively 
current of water over the surface of the marsupium, which lies immedi- 
ately inside of them. Finally, in the genus Truncilla, the inner edge of 
the mantle, which also has papille, is removed from the outer edge, 
thus enclosing a separate compartment, which may possibly be regarded 
as a kind of reservoir. 
Thus it becomes evident that of these four arrangements acquired by 
the Lampsiline to provide breathing water for the glochidia, the last 
was the most advantageous, because it included the possibility of 
further development and improvements such as we find realized in 
the various genera just mentioned. 
5. There remains yet a fifth type of structure, that found in Frier- 
sonia. Here the marsupium is truly lampsiline, resembling somewhat 
the Obovaria-type, but it is not so swollen, and instead of being blunt, 
