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. 3 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 Planorbis 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 Uniomdcz, 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 is 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 maxi- 



