288 



mum record. Their fluctuations are erratic, and show no apparent 

 relation to hydrographic or other environmental changes. 



Lampsilus anodontoides (Lea) Baker. Glochidia referred with 

 some uncertainty to this species appeared somewhat irregularly in 

 the plankton in small numbers in September-December and again 

 in June-July. The seasonal distribution in two periods suggests 

 the inclusion of two species. 



Arcidens confragosus (Say) Simpson. Glochidia of the type 

 referred by Lea to the old genus Margaritana, and presumably 

 belonging to this the commonest member of this genus (as formerly 

 understood) in our locality, were taken in the plankton December 

 18, 1895, in small numbers. 



BRYOZOA. 



This group is represented in our plankton by the floating stato- 

 blasts, when these occur, as in Pectinatella and Plumatella, by 

 detached and floating fragments, as in Urnatella, or by natant 

 colonies, as in Lophopus and Cristatella. Genera such as Fredericella 

 and Paludicella, whose statoblasts sink, fail to appear in the plank- 

 ton, though in some cases they may be abundant in the bottom 

 fauna. The Bryozoa are plankton feeders, and play an important 

 role as plankton reducers in vegetation-rich backwaters. 



DISCUSSION OF SPECIES OF BRYOZOA. 



Cristatella mucedo Cuvier. This species was found in the back- 

 waters in summer months, especially in Quiver Lake. Statoblasts 

 probably referable to this species occurred sparingly in May and 

 August. 



Lophopus cristallinus Pallas. This rare bryozoan occurred in 

 the channel plankton, though not in our quantitative collections, in 

 July, 1897, in that part of the channel containing the discharge from 

 Quiver Lake. Small, free-swimming colonies of 5-50 zooids were 

 taken in surface waters. 



Pectinatella magnified Leidy. Statoblasts of this superb bryo- 

 zoan were not uncommon in the backwaters, and were seen several 

 times in the vernal plankton of the channel. The large floating 

 colonies are found near the surface in July-October in the open 

 backwaters, and more rarely in the river itself. The translucent 



