Ortmann: Families and Genera of Najades. 223 



The main figure always represents the sofl parts seen from the lefl 

 >idr, with the left half of the mantle removed. 



The Najades have been divided into three families: Margaritanidce, 

 Unionida, Mutclidcr. The first is holarctic; the second is known from 

 Eurasia and North America, hut probably exists also in Africa; the 

 third is restricted to Africa and South America. 1 



Family I. MARGARn anid.e Ortmann. 

 Diaphragm incomplete, formed only by the gills: outer lamina of 

 outer trills only in part connected with the mantle, posteriorly free 

 for a considerable distance. Anterior end of inner gills separated 

 from the palpi by a wide gap. The margins of the mantle do not unite 

 or approach each other anywhere, and there is no tendency to form 

 branchial and anal siphons, and no supra-anal opening is present. 

 Gills without water-tubes, interlamellar connections irregularly scat- 

 tered, or forming irregular, oblique rows, or incomplete septa, which 

 run obliquely to the direction of the gill-filaments. Marsupium formed 

 by all four gills. Glochidia small, semicircular and globular, without 

 hooks, but with irregular small teeth at the ventral margin. 



Family II. Unionid/e Swainson (restricted). 

 Diaphragm complete, formed only by the gills: the outer lamina of 

 the outer gills connected with the mantle at its posterior end. An- 

 terior end of inner gills separated from the palpi by a more or less wide 

 gap. Margins of the mantle drawn together by the gill-diaphragm, 

 but not united, thus separating the anal from the branchial opening, 

 and the anal is generally closed above by the union of the margins of 

 the mantle (it rarely remains open), and, when closed, it always 

 leaves a supra-anal opening (which is very rarely obliterated). Gills 

 always with water-tubes, formed by interlamellar connections de- 

 veloped as continuous septa, running parallel to the gill-filaments. 

 Marsupium formed by all four gills, or by the outer gills alone, or 

 by parts of the outer gills. Glochidia of various shapes, suboval, 

 subtriangular, or celt-shaped, with or without hooks on the ventral 

 margin. 



1 The writer is convinced that the Najades will prove to be a most important 

 group for the reconstruction of the ancient geographical features of the earth. 

 As long a? our knowledge of the systematic relations was obscure, or even dire< tlj 

 wrong, any attempt in this direction must have been a failure. 



