Ortmann: Families and Genera ob Najades. 225 



in contacl with the palpi. Branchial and anal siphon sharply sepa- 

 rated by the union of the margins ol the mantle. Anal opening open, 

 or closed above, in the latter case withoul forming a supra anal open- 

 ing. In some genera the margins of the mantle unite also in front of 

 the branchial opening. Gills with very indistinct intercommunicating 

 water-tubes, and interrupted interlamellar connections; or with well- 

 developed water-tubes and septa, parallel to the filaments. Mar- 

 supium formed only l>y the inner gills. The larvae are glochidia or 

 lasidia. 



This family is divided into two subfamilies. 



I. Subfamily IIykiin.k Ortmann. 

 Anal opening closed above. Marsupium with septa-like, inter- 

 rupted, interlamellar connections, forming incomplete, communicating 

 water-tubes. Non-marsupial gills with poorly developed interlamellar 

 connections. Larva a glochidium. 



2. Subfamily Ml TELIN.E Ortmann. 

 Anal opening open or closed. Marsupium with well-developed, 

 continuous septa, forming well-defined water-tubes; also non-marsupial 

 gills with septa and water-tubes. Larva a lasidium (?). 



There is no doubt, that of these three families that of the Margari- 

 tanidtz is the most ancient; the lack of any tendency to form siphons, 

 the incomplete diaphragm, the absence of real septa and water-tubes 

 in the gills, the absence of a division of function in the gills (all four 

 gills in the female are used both for breathing and for receiving the 

 eggs), are peculiarities, which establish the primitive character of this 

 family. 



The forward step in the development of the Unionida consists 

 chiefly in the specialization of the marsupial structure. In the most 

 primitive forms, all four gills are used as marsupia, but later on a 

 division of labor is effected, so that in the female some gills serve only 

 the purpose of respiration, while others, or parts of them, become 

 organs used in propagation. But always, in this family, the inner 

 structure of the gills is more complex than in the Margaritanida, 

 which is expressed, by the development of septa and water canals; 3 



3 In Margaritana monodonta a slight tendency is shown to develop septa, but 

 here the septa are entirely different from those of the I r nionid<B, not running parallel 

 to the gill-filaments, as in the latter, but diagonally to them. 



