ORTMANN: FAMILIES AND GENERA OF NAJADES. 301 
genera (Pseudospatha, Hyriopsis, Chamberlainia, Cristaria, Lepido- 
desma, Pilsbryoconcha), but of all these the soft parts are unknown, 
and we may entertain strong doubts as to their belonging here. 
In order to understand the structure of the Lampsiline, and their 
further differentiation, we must recall their essential characteristics 
and their purpose. We have seen that the soft parts are accommo- 
dated to two functions: (1) owing to the extended breeding season 
(in bradytictic forms) that of securing the proper aération of the gravid 
marsupium, (2) the discharge of the glochidia through the edge of the 
marsupium. The latter physiological character is unique, and is 
found only in this subfamily. The former occurs in the Anodontine, 
but we have seen that it is there brought about in another way. 
Very likely the adaptations to these two peculiar functions are con- 
nected in a degree. We see that it is the general tendency among the 
Lampsiling to move the marsupium toward the edge of the gill, and 
even beyond the latter. This has the effect that it is removed, more 
or less, from the natural outlets, and comes in close contact with the 
outer water flowing over the gills. Under these conditions it is easily 
understood that the habit was acquired to discharge the glochidia 
not by the long way (the suprabranchial canals), but by the shortest, 
by making them go through holes in the edge of the marsupium (see 
Plate XVIII, fig. 10). Thus we may say that the lampsiline mar- 
supium serves two purposes, and is built according to a type which 
meets first the necessity of aérating the marsupium, and which in con- 
sequence of the structure so assumed, made another peculiar way of 
discharge desirable. 
To supply breathing water for the glochidia, however, is of chief 
importance, and thus the further differentiation within this subfamily 
is easily understood, when keeping this point in view. Allowance 
should be made for certain expressions used in the following statement, 
and they should be excused by my desire to make the facts as clear 
as possible. 
Among the Lampsiline, there are at least four types of marsupial 
structure, which represent as many different attempts to find a way 
of supplying breathing water to the marsupium. They all agree in 
having as a common feature the extrusion of the marsupium beyond 
the edge of the gill, and its investiture by only a very thin membrane, 
so that osmotic processes are greatly facilitated. In addition, in 
three of these types, there is also developed the tendency to move the 
