Ortmann: Families and Genera of Najades. 303 



branchus, but I think it has been independently acquired, and does 

 inn indicate close relationship. 



4. In the fourth type (all other genera), an entirelj differenl ar- 

 rangement to provide aeration for the marsupium has been eft < 

 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. But a 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 (Obovariu, Nephronajds, Amygdalonajas, Plu- 

 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, Lump- 

 si! is), this part of the inner edge of the mantle is greatly developed, and 

 carries special appendages in the shape of strong papillae 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 papilla?, 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 Lumpsiliiicc 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 nol so swollen, and instead of being blunt. 



