287 
mus appeared. They were recorded in all months but February 
and December, though 20 of the 30 records and 81 per cent. of the 
individuals appeared in May—August. The numbers are never very 
large, the maximum record, 5,424 per m.* on June 29, 1894, being 
due to a number of fragments of egg-strings. 
MOLLUSCA. 
GASTROPODA. 
The adults and young of many of our aquatic gastropods have 
the habit of gliding on the under side of the surface film of water, and 
they are also frequently dislodged from their foothold on aquatic 
vegetation, and thus enter the habitat of the plankton temporarily. 
This is especially true in vegetation-rich backwaters. The smaller 
forms, such as Ancylus, Amnicola, and Planorbts parvus were occa- 
sionally taken in the summer plankton of the channel. 
LAMELLIBRANCHIATA. 
This group is represented in the plankton by the larval stages, 
or glochidia, of the Untonide, which form an important part of the 
bottom fauna of the stream and its tributaries. 
Anodonta corpulenta Cooper.—Average number of glochidia, 21. 
The seasonal distribution of the glochidia in the plankton is very 
well defined. With but two exceptions the 48 occurrences all fall in 
October—April, and 40 of them in November—March. The occur- 
rences are thus during the period of minimum temperatures; indeed, 
31 of the 48 are at temperatures not exceeding 35° in surface waters, 
and only 9 are above 45°. The earliest autumnal record 1s Septem- 
ber 30, at 58°, and the latest vernal one, June 6, at 79°. Generally 
the earliest records are in the closing days of September or the early 
ones of October, and the latest records are about the first of April. 
The occurrences are more frequent in December—March, the glo- 
chidia appearing in 64, 50, 53, and 60 per cent. of the collections, 
respectively, in these months. Their numbers are also several 
fold greater at this season than in the earlier and later months of 
their occurrence. The period of minimum temperatures is thus 
the season of greatest discharge of glochidia. The numbers are 
always relatively small, 520 on December 28, 1897, being the max1- 
