Ortmann; Families and Genera of Najades. ."><»i 



genera i Pseudospatha, Hyriopsis, Chamber iainia, Cristaria, Lepido- 

 desma, Pilsbryoconcha), bul of all these the sofl parts are unknown, 

 and we may entertain strong doubts as to their belonging here. 



In order to understand the structure of the LampsilinCB, and their 

 further differentiation, we must recti! their essential characteristics 

 and their purpose. We have seen that the soft parts are accommo- 

 dated to two functions: (i) owing to the extended breeding season 

 (in bradytictic forms) that of securing the proper aeration 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 Anodontince, 

 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 

 LampsilinCB 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 aerating 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 LampsilinCB, 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 



