ORTMANN: FAMILIES AND GENERA OF NAJADES. 226 
in contact with the palpi. Branchial and anal siphon sharply sepa- 
rated by the union of the margins of the mantle. Anal opening open, 
or closed above, in the latter case without 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 by the inner gills. The Jarve are glochidia or 
lasidia. 
This family is divided into two subfamilies. 
1. Subfamily HyriiNn# 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 MUTELIN2 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- 
tanide 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 Unionide@ 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 Margaritanide, 
which is expressed, by the development of septa and water canals;? 
3In Margaritana monodonta a slight tendency is shown to develop septa, but 
here the septa are entirely different from those of the Unionidae, not running parallel 
to the gill-filaments, as in the latter, but diagonally to them. 
