Ortmann; Families \m> Genera oi Najades. 279 



the glochidia varies somewhal 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 in a direction transverse to the gill. There are also further 

 peculiarities in the marupium of this ;j,emis, 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 >hells sometimes supplemented 1>\ 

 minor characters of the soft parts. 



The shell, in this subfamily, is epiite 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 left 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: 



ai. 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. 



c\. Beak-sculpture not tubercular, and no sculpture upon the disk, but 



sometimes upon the posterior slope Symphynota. 



ci. Beak-sculpture tubercular, and strong tubercles and other sculpture 



upon the disk Arcidens. 



bt. Hinge-teeth completely absent. Shell smooth and thin Anodonta. 



a-- Beak-sculpture concentric, fine. Hinge-teeth practically absent. Mantle- 



24 It is possible that the structural differences of Slrophilus are connected with 

 the fact recently discovered by Lefevre and Curtis (191 1) that the glochidia of this 

 genus do nol pass through a parasitic stage on fishes. 



