ORTMANN: FAMILIES AND GENERA OF NAJADES. 223 
The main figure always represents the soft parts seen from the left 
side, with the left half of the mantle removed. 
The Najades have been divided into three families: Margaritanide, 
Unionide, Mutelide. The first is holarctic; the second is known from 
Eurasia and North America, but probably exists also in Africa; the 
third is restricted to Africa and South America.! 
Family I. MARGARITANIDZ Ortmann. 
Diaphragm incomplete, formed only by the gills: outer lamina of 
outer gills 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. Unronrp# 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. 
1The 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 as our knowledge of the systematic relations was obscure, or even directly 
wrong, any attempt in this direction must have been a failure. 
