ORTMANN: FAMILIES AND GENERA OF NAJADES. 279 
the glochidia varies somewhat in the different species (see Plate XIX, 
figs. 2, 3, 4). In only one genus, Strophitus, the anodontine-structure 
of the marsupium has undergone a marked change, and has made a 
step in advance. Here each ovisac, which remains simple in other 
genera, is subdivided into a number of secondary compartments run- 
ning ina direction transverse to the gill. There are also further 
peculiarities in the marupium of this genus, which concern the mutual 
cohesion of the eggs and glochidia.** 
Strophitus is the only genus, which must be separated from the rest 
on solely anatomical grounds. In the case of all other genera the only 
criteria are the differences of the shells sometimes supplemented by 
minor characters of the soft parts. 
The shell, in this subfamily, is quite variable in shape, but in most 
forms it is not very thick, or it is even quite thin. The hinge is ex- 
tremely variable, showing all stages from a complete development to 
complete reduction. If present, the hinge-teeth are peculiar: the 
pseudocardinals are supplemented, in the lett valve by an additional 
(generally a third) tooth, which is formed by a projection of the 
interdentum, and extends below the corresponding part of the right 
valve. Sometimes this supplementary tooth is rudimentary, and it 
may be well separated from the posterior pseudocardinal, or may 
be fused with it. 
A very important feature of the shell seems to me the beak-sculpture, 
which indicates two, possibly three, lines of development within this 
sub-family. 
We would thus obtain the following scheme of classification: 
a1. Beak-sculpture double-looped, with a more or less sharp sinuation or re-entering 
angle. Mantle connection between anal and supra-anal moderate or 
very long. No tendency to unite the inner lamina of inner gills with 
abdominal sac. 
b}. Hinge-teeth more or less developed, at least pseudocardinals present. 
Shell not very thin. 
ci. Beak-sculpture not tubercular, and no sculpture upon the disk, but 
sometimes mpon’ the posterior slope..42-..-..0...--.4- Symphynota. 
c2. Beak-sculpture tubercular, and strong tubercles and other sculpture 
UWP OMUMeIGIS ewan te rieeeseys ors Siniens: Sen ORIN naoeeh voce wel sie cheraeler Arcidens. 
bo. Hinge-teeth completely absent. Shell smooth and thin......... Anodonta. 
ax. Beak-sculpture concentric, fine. Hinge-teeth practically absent. Mantle- 
24 It is possible that the structural differences of Strophitus are connected with 
the fact recently discovered by Lefevre and Curtis (1911) that the glochidia of this 
genus do not pass through a parasitic stage on fishes. 
